Deep lines on Y axis while engraving

Hey everyone! I’ve been using my neje master 2s max 40w for two months without any problem, but today after a 2hr job I started getting some lines on wide engravings on Y axis.

I’m using latest lightburn and my fill settings are 120mm/s 90% pwr, tried changing the fill offset but no luck, also tried tightening the belts just in case but nothing.

any idea will be appreciated!

1cm squares engrave without any issues

it was bad belts all along, i changed them to ones with reinforced wires and everything is working fine now!