Just got a diamond drag bit and have been trying it out on polycarbonate. 1/4" from lowes. My problem has been inconsistent engraving depths. Is this due to the polycarbonate thickness or quality? I started at .015 pressure and has areas that were completely missed. Increased to .025 and got full coverage but you can see where there were lighter passes.
All of the diamond drag bits have a set screw that sets the spring tension. Did you adjust the spring tension to get a consistent pressure. I would think that too light of pressure may be your problem and/or the diamond part of the bit is not moving freely and a disassembly might be in order to see if you have any burrs or containment keeping the bit from moving freely.
Is your machine trammed, and wasteboard surfaced? If it is not surfaced, this may be issue, as there will be high/low spots.