I have been using the S70 with 650/75 parameters but overnight it stopped cutting 3mm and 6mm in one pass without changing anything. Now I had to adjust the speed to 300 and increase the power to 100 so it cuts in one pass.
I have checked the air pump and it is clean, the flow is the highest, Lenses are cleaned , no obstruction on the nuzzle, cleaned the rails and made sure everything is tight according to the manufacture instructions. .
The only thing I did , that I can think of, was to update LightBurn to the newest versionbefore this started happening.
Roll back the last update, it’s the fastest and easiest way to test if it’s LB.
Find the latest, for you working version and install it over your current version of LightBurn.
If it is the same material you cannot cut with the old parameters, the lens is not broken and clean, then only your laser (diode) remains as a source of error.
(I sometimes experience glue vations in plywood and cannot cut it properly, that is why I mention the material)