I also have a p-cut 630 and it took me a long time to obtain good cuts, Then I acquired a Roland GX-24 and couldn't believe the difference in the cut of small letters. The Roland has a vacuum on the vinyl that keeps it from lifting.
Using a paper spring clamp I add a little weight to the leading edge of the vinyl on the P-Cut to combat the lifting and it made a huge difference. Once the vinyl is near the floor it's own weight seems to be enough.
Every one is right about the blade adjustment, my P-Cut is very sensitive to proper blade adjustment also. I believe having only the tip of the blade exposed helps to hold the vinyl down. I also found that really cheap vinyl is hard to cut on the P-Cut.
