Ahh....one of those huh? It helps if you read the manual..
Originally Posted by Sean
If you are getting a "hook" at the corners, there are two possibilities, your blade is not deep enough or it is too tight against the sliding support. To set the blade deeper, turn the top screw counter clock wise. To separate the blade from the sliding support there is a little screw on the base that you press down, turn it CCW too.
You should have the blade deep enough so that it slightly scores you support mat, and it should be just right against the sliding support or the blade gets pushed out and results in the "hook". I had the same problems and had to fiddle with the depth and "press" until I got it right.