Are you printing and applying full bleed prints? If so, and there's no way around it, the longer you let the print sit the more secure the edges will be. Ink to the edge can tend to contract the vinyl a bit unless the print is a couple of days old.
Design permitting, if you can leave a white unprinted border, maybe 1/4" plus or minus, the edges will stay down far better than a full bleed. Also if you can do round corners, these will stay down better as well. With a square corner there necessarily exists a point at which the vinyl is thicker than it is wide. It's at this point that the adhesive is marginal and if it's going to lift, that's probably where it will start. Not so much with round corners.