For solid fills it’s going to raster engrave them. The depth of the engraving is going to be determined by a number of factors. For machine settings, engraving depth is going to be determined by scan speed, power, and scanline steps. Slower speed and/or higher power will engrave “harder”.
Scanline steps determines how far the laser moves down for each following raster scanline. The K40 laser beam typically has a minimum diameter of ~0.15 mm at the focal point. So having a scanline step of say 0.05mm(0.002in) means you are getting a ~3x overlap on area of the engrave. Smaller scanline steps gives “harder” engraving since it’s in effect giving more “passes”, but it comes at the cost of longer engrave times.
I think K40whisperer has the default scanline step at 0.05mm(0.002in). I would suggest playing with speed and power first.
What settings were you using? Also a pic of your engraving may help as well.