Pressure is too weak: increase it. DO NOT BE AFRAID TO INCREASE THE PRESSURE TO 300+!
The mat is drooping due to paper weight or humidity. Place a small box directly behind the cutter to keep the mat flat or use our Creative Cutter platforms. 
Cutting project is outside of allowable margins. Move it or check page settings.
There is a communication error between machine and computer (often an HPGL #1 error). Restart both and check cord connections.