It is pretty normal to have parts of the edge of the wafer peel up. It happened to me a good bit when I was sampling Coloplast products, and still does from time to time with my Convatec set up. Like Pluot said, a good solution is to clip off the part you know always peels up, and see if this keeps it from happening more. You could also try a product called "sure seals". It is a thin plastic sheet that sticks over your wafer, extending beyond the edges. It helps keep water out and the edges stuck down. Here is the link:
www.alpglobal.com/Sure-Seal-TM-Rings.html