The PopGrip’s gel is designed to stick to most materials, but it sometimes has issue with sticking to silicone and waterproof cases and anything with too much texture.
If the gel on the back of your PopGrip gets dirty, it may not stick as well. Just give it a quick rinse and let it dry. Remember, your PopGrip shouldn't be exposed to air for more than 15 minutes or the gel will dry out. Watch this video to see how to reposition your PopGrip.