Why are my Invisalign trays starting to push away from the surface of my two front teeth?

At the end of my last set of trays I noticed the tray at gum line of my two front teeth was pushing away from the surface of my teeth and leaving a bit of a gap between it and the tray. When I started tray 11 it was sitting flat on my teeth again. However, a few days of wearing it and it has starting to do the same thing as the last tray. I have two attachments on my both my front teeth and my trays are seating well and sitting flush otherwise. Is this normal?