I have this fancy new mat cutter, very similar to a Logan 750, but all aluminum and able to do up to 40" cuts. The cutter has a groove that slips over a rail so you cut straight. However, I can't get straight corners. About 1/2" out, the blade drifts inward and slightly wavy. I figure it's got to be the mat board drifting, but I don't see what I'm doing wrong. Has anyone else experienced this problem?