Nils1990, I realize now, that I gave you wrong information, about how to correct your problem with air bubbles. What I told you would help if you had a denting problem, but not with trapped air. What slowfish told you is very much what you are probably needing to check. It does look like trapped air, instead of several dents.
Sorry , I would not intentially send you in the wrong direction.
Hope you get it lined out.