Could be a couple things. If you run a lot of magnetic you may have worn down the grip of the drive rolls.
Also the middle pinch rollers don't do much. It is the outer pinch wheels with a specific taper to keep vinyl flat. Those are a consumable. Just had mine replaced since we were running out about 0.150" to 0.180" in 8'.
The other thing is do not assume that just because you use the sheet cut that it is square to the roll. It will not leave the edge 90 degrees after you run a job. Short runs you can get away with lining up the edge to the platen, but at some point you must re-square the roll.
My wife ran a number of small jobs one right after another then couldn't figure out why the contour cut ruined the decals. I pulled the roll and used a t-square and found the roll was 0.500" off. She said: Well I've been using the sheet cut every time so it should be perfect". Nope.