If you're having issues charging it's likely one of these 3 reasons:
Your case is too thick. We recommend cases under 2.2mm thick - most are thinner unless it feels extra chunky. If charging works with Snap Grip on your bare phone, it's probably your case.
You're using a non-MagSafe charger. The alignment of your phone and the charger is very important. Placing your phone onto a pad charger has a much lower success rate than using an auto-aligning charger.