Laser doesn't seem to be burning hot enough anymore

Hi,
Newbie here. Been using lightburn for about 2 months. I am using gimp and/or inkscape along with lightburn on an OLM2 20w. I am trying to use the Norton method to engrave some tile coasters. (Whitepaint on white tile) The problem is, it is requiring really low speeds and very high power where, a month ago, not so much.
I have attached some test plates to show the difference, but incase the settings are hard to read, the current test plate is showing the ideal ranges to be 90%-100% power at 250mm/min - 750mm/min whereas my test plate from a month ago shows speeds of 1750mm/min to 2000mm/min at those same power levels. I’d rather not run my machine at those power levels to save on wear and tear of my laser head, so I was running at 85% power at 1750mm/min on the old project, which turned out beautiful, but was low intensity on the actual quantity of engraving. (Beatles white album, haha)
I am fully open to the possibility of user error.
I’ve also noticed a shift in intensity in some of the pieces I’ve burnt. Scanning horizontally (0°) the image is lighter on the left, than it is on the right. Not sure if it’s related?

Just to provide as complete information as possible: the tiles are from same brand and batch, all sprayed with paint from the same can. (Rustoleum 2x matte white) With the same application method. (2 light coats sprayed perpendicular to each other. I also