What should I do if my skins doesn't stick anymore ?

If your skins don't stick anymore, it's because they are dirty (or wet). You should clean them with dish soap or dedicated products. Swix Skin Cleaner is a great place to start.

If you're using skins multiple times per day, it's a good idea to keep them warm when you're skiing. Keeping them inside your jacket for the descent can really help as it dries the moisture and ensures the glue works when you put them back on.