What bit are you using (type, diam, length) and approx RPM? What spindle, how long is your zaxis, how big is your machine?
Chances are really good you are spinning the bit too fast and taking way too small of a bite, causing the bit to just melt its way through the part, a larger bite means the bit will take the heated plastic out of the cut, metal is the same way, but is even harder to cut because too small of a bite and you will work harden it and break
A single flute bit lets you spin it twice as fast as a two flute bit while taking the same size bit out of the material. We have a high rpm spindle single flutes bits are best and still slowing it down helps. If you can’t slow it down you need to move it faster. To move it faster you might need to take a shallower depth of cut to not cause too much tool deflection.
An upcut can pull the sheet from the table so unless you have it double sided taped down you should use shallow passes, just watch for the material to vibrate up and down and you know your too deep. A down cut can push all the material to the bottom and melt it, these are best with through cuts.