Most of the time gum and bone underneath shrinks over time and denture is not as fit as it use to be. You dentist can reline the denture for you to achieve a good fit again.
It can have couple of reasons:
Simply whatever works for you is the best for you. A very common type is Polydent.
We strongly suggest that you do not do that. It can cause faster bone resorption over time and also encourage bacterial and fungal infections on your gum.