My 2nd tray of clear correct didn't fit properly along my back right top teeth.There was a gap between my teeth and the plastic that my bottom teeth would collide with when I talked. My dentist told me my teeth would move and it would be fine.I got my 3rd tray and it also didn't fit.I can pull the back edge easily with my tongue.when I drink from a water bottle the back right portion flaps up and down.No other parts of my retainer are doing this.Is this normal for clear correct to fit like this?