Has the pad stuck to the tone hole and stuck to the cup?
If so be careful to mark the pad so you don't turn it, must go back in the cup the same way around, otherwise it won't seal again and will have to be replaced.
Guessing it's not got enough glue from new. If the glue is clear, glue gun is OK, just make sure the ring on the pad from the tone hole lines up properly with the tone hole. You may be able to warm the cup with a lighter to soften/restick the pad. Also make sure the pad sits properly in the cup and sits correctly on the tone hole chimney.