Two things:
One you need to adjust your contrast so that you get a really high dynamic range. If you don’t know how to use curves, now is the time to learn. You want your engraved areas to range from pure black to pure white for the greatest height differentiation.
Two, slow or way down. Like speed 150. You’ll char it a good bit but get depth for sure.
After that it’s a matter of fooling with other settings like lpi and multiple passes. Engraving is easily the most complicated thing we do with the Glowforge.