The most common reason for this is trying to go faster than your laser is allowed to. LaserGRBL uses ‘constant power’ mode by default, which means when the laser is on, it’s on at exactly the power level you ask for, always.
LightBurn uses ‘variable power’ mode by default, which means that the laser goes from 0% power when it is not moving to the power level you ask for when it reaches the requested speed. If you asked for 100% power at 3000 mm/minute, then as the laser accelerates, the power is increased from 0 to 100% as the speed increases from 0 to 3000.
The problem arises when you ask the laser to go faster than the speed limit set in the firmware. If you requested 6000 mm/minute, but your laser had a maximum allowed speed of 3000, you would never reach the requested speed, and therefore you will also not reach the requested power level.
If you ask for 6000mm/sec in LaserGRBL, but your hardware can’t go that fast, it will only go as fast as it is allowed, but by default, it does not ramp the power. You can get identical or better results in LightBurn by simply asking for a speed that is at or less than the speed limit of the machine, because it is pointless to ask it to go any faster.
You can also change the layer to use Constant Power mode in LightBurn, which will mimic the behavior of LaserGRBL. You’ll get burnt spots at the start / stop points of the design, as well as sharp corners, which is why we don’t use this by default.
We have a FAQ entry that covers this here: