The PopSocket adhesive is designed to stick to most materials but not all. PopSockets like a smooth, hard, flat surface and sometimes have trouble sticking to silicone, waterproof coatings, rough textures or grooves and leather.
The good news is that the gel on the back of your PopSocket is designed to be reactivated (I know, awesome)! Rinse the base of your PopSocket with water (no soap), let is air dry for 10 minutes but don't leave it unattached to a surface for more than 15-30 minutes or it might lose its sticking power forever. This should get you Popping again!