After some back and forth with the dealer it was the PSU, bought a new one, setting it up today - will update in a little while 
If you go to Edit > Machine Settings, then all the way to the bottom in the Vendor Settings area (unfold it) there’s a section called ‘Laser’ and in there are Min & Max power values. It’s possible that the factory set yours to less than 100% to prevent you from nuking the tube.
Hi Oz,
That’s not it, laser setup is all good. I looked at the PWM with a scope and all looks well.
In the end, I now have the LPSU adjusted and the laser is working fine, gray scale engravings are now working well with the laser power being more linear.
Thanks
Joel
Ok, replaced the PSU and… same 
Put a 4mm piece of acrylic, speed 12mm, power 95%… 10 passes to cut a circle and still it didn’t cut through!
Checked the laser tube, there are no cracks or damages… how else to check if it’s damaged?
what else could be wrong?
Stilll waiting for Amazon to deliver my am meter 
What is the Min power setting when you do this?
Sounds like the tube is not outputting what you think it’s outputting.
The only way to know 100% is to test the beam using one of these exactly as @Bonjour suggested above
Cheers
Sasquatch
I set both min and max to 95% just to be sure…
I bought one of these:
https://www.amazon.co.uk/gp/product/B07CZ1MNXZ/
Should arrive in the next 6 days…
@NewbDudey. That’s only going to help you monitor your ability to identify if you are overpowering your laser. You still need to be able to identify that your laser is outputting the wattage you’re tube is ratted to output at. The only way to gauge the lasers true output power is by using the device I and @Bonjour suggested you use. This device absorbs the lasers beam and tell you the exact output power of what your laser is emitting.
Cheers
Sasquatch
@Sasquatch, that’s an expensive diagnostic tool. Maybe I buy one and make it available for rent (with a deposit of course). 
I do want one of those toys for myself though.
Compared to the cost of our machines it’s peanuts 
@Stroonzo.
They do have a great selling point. Warranty replacement if the wattage output is not consistent for the duration of their warranty period. From CO2 manufacturer or machine manufacturer.
I do like the rental option angle. I will definitely do that. After all that business model is COVID19 friendly. 
My meter paid for itself whenI ordered a replacement 40W tube for a K40 and the output was 32W (Which I knew it would be) Ebay forced the seller to refund my money for “not as advertised” reasons and let me keep the tube. My little bit of revenge on the shady sellers who over promise and under deliver.
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Not really. My tubes cost ~$1000 to replace. This tool will last decades.
The alternative is to hire someone like me to come and fettle your laser, and I charge $120 an hour.
Without a meter you are only guessing.
There’s so much more to it than just reading the power output. For example:
Use it between the tube and first mirror to give you initial tube reading, along with an RMS multimeter that measures the precise output at a specific, precise mA.
Move to the next position after the first mirror, and you can read your mirror losses. Same for next position, then remove your lens and read at the final position to give you a reading of your tube and mirrors.
Take readings all over the extents of the bed and you can get a better understanding of your mirror and chassis alignment.
And, of course, it allows you to keep track of your tube power over time.
If, for example, your machine suddenly loses power, it can take just a couple of minutes to see where that loss is occurring without having to guess, or fiddle and potentially cause some other issue.
If course if you have a $300 K40 or 5W diode, it is a significant investment.
As far as tools go, I have a hammer that cost nearly as much and a spanner that cost twice that. Bell-Laser are not a price-gouging site and they hand-make all their meters in the US. The process of calibration is not insignificant.
Go look at some hand-made tools from the US and then compare the price.
There are other, cheaper tools - plastic, digital displays, etc. but they don’t have the reliability or calibration simplicity of the Mahoney. It’s so basic, it will work optimally, forever.
The Model T of the laser meter world. 
An ammeter is half your diagnostic.
It only shows what power is coming from the PSU.
If you are, for example, showing 25mA at your ammeter, but you can’t cut paper, you are no wiser as to the reason.
It could be a faulty tube, dirty tube, faulty or dirty mirrors or lens, alignment, mechanical, whatever - all you know is the power output of your PSU.
People put so much faith in an uncalibrated Chinese ammeter - I’ve never worked out why. I can’t say I have ever used the one installed on all of my machines.
A/ They’re unreliable - I have read differences of nearly 5mA from real, calibrated tool readings. That’s enough to destroy a tube when you think you’re putting out 18mA and you’re really putting out 23.
2/ Why? What do you think you are you reading and what do you think it is telling you?
iii/ So you can see it is putting out ~20mA - so what? How does that change what you are doing?
I have a dial just below mine on my latest machine. I use that somewhat regularly to run a second pass, or to fix a problem - I can turn power off entirely until I reach the point I want to resume cutting, or I can run a light second pass to compensate for environmental conditions, etc. - but as to the meter itself, I guess it shows I’m putting out something.
I prefer to calibrate the machine from new, then I know what the upper limit is and can happily tell the software to run flat out at 100% and be sure it’s not going to be damaging the tube.
I use a screwdriver and a multimeter and the Mahoney power probe.
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I wasn’t serious. Just having fun. It really is not expensive.
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Thanks for the replies guys! That will prob be my next buy
Still learning 
Calibrating an ammeter is no big challenge
But using a co2 laser machine (K40 / 50 …) without ammeter is a challenge that ends up with a much shorter life of one’s tube.
And even with a 10% deviation from a Chinese ammeter, it will show “good enough” for our purposes.
Skål
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Seems like a dial is also a good idea… to quickly do a second pass with lower settings… on the other hand… that isn’t a biggie
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I have one of these. Don’t know if they’re any better or worse than the others. Made by Macken.
https://www.kenteklaserstore.com/digital-laser-power-probe-kit.aspx