Laser skipping parts of the image, leaving empty horizontal lines

Hi, I finished building a new laser few weeks ago based on this design Laser Engraver 15watts by infectedfpv26 - Thingiverse
I’m using a Sculpfun S9 5.5w laser, arduino uno + cnc shield running GRBL 1.1
So far everything has been working fine, maybe except stepper motors running very hot (salvaged old Anet A8 Nema17 motors). I’ve succesfuly engraved one picture on a 80x30cm wooden board but now that I’m trying a diferent one I’m being left with a lot of areas not getting engraved at random parts of the image.

The laser seems to slow down a lot in these areas, and while the beam is still on, it doesn’t have enough power to leave a mark. Sometimes it looks like it’s trying to catch up and burns the last bit of the line a lot deeper.

I tried using lightburn in passthrough mode, with the image prepped in imagr, at few diferent DPIs and speeds and also just uploading image to lightburn to let it do the proccesing (jarvis, dither, speeds from 2000 to 3500) and I’m still getting the same results. The belts are tight, board is level with the laser head.
After reading up similar topics my best guess is arduino and cnc shield bandwidth issue, but maybe (hopefully) I’m wrong.

All right, I found the problem. Everything was caused by a 3d printed air assist nozzle that I mounted without a proper o-ring that caused air to leak. Wood was treated with borax and without enough pressure going through the nozzle the residue kept building up around it and started blocking the beam, after the nozzle was nearly completely clogged it would randomly shoot out the residue and engrave the rest of the image without a problem.
Fixed the seal and everything works fine :upside_down_face: