My lip injections are gone after a month every time?

I have gotten lip fillers (Juvederm and Restylane) 6 times since the end of August. They are good for two weeks and then they gradually get smaller, and are gone after a month. My Doctor says she has never tried anything like this and says it's very weird. She says that when i come back after a month she can feel that there's still something in my lips but it's not visible at all. Is there anything I can do or will my lips never accept the filler?