Thing is, the center plys will probably have occasional voids in them and different manufacturers use different compounds to fill them. I have seen Bondo body filler in them, and that will not cut.
So before you move the work after an attempted cut try to lift the cut out with a piece of tape or a pick, and if the material hasn’t moved you can run the file again.
Just be aware that you will run into that filler problem at some point. That’s one of the attractive points of the proofgrade ply with an homogeneous MDF core.
I’m currently running a production job of 1000 tokens engraved on both sides, and because:proofgrade: is already sanded, and has a finish on it, it’s a no-brainer time and frustration saver.
I wish the company would offer the ply unmasked for a bit of savings. On a job like this one I pull the mask off before running the file, household ammonia wipes the smoke stain right off, saving me about a week of weeding both sides of 1000 pieces.