My upper front flipper became loose within two weeks. My dentist said there was nothing he could do other then to apply denture cream. I have been using Fixodent and it does help keep it in place while my bone graft is healing.
Flippers are by definition temporary (true name for a flipper is a temporary partial denture). They are not meant to last more than about 3-6 months. You can take them back to your dentist and possibly relined or remade, but you should find a permanent solution (assuming you aren't already in the process of one...ie implants)