Hi,
so you don´t have "spannen", but "Blasen" (bubbles) on your Rubber (Sponge and rubber losing contact at a small or sometimes bigger area).
"Spannen" means to stretch the rubber, as this is one reason for those bubbles. And on the Joola-Website there is a warning not to stretch the tensor-rubbers, because this could cause such problems as you have right now.
So, as you don´t glue and stretch your rubber, where do those "bubbles" appear? Always in center, where most contact with the ball is made?
Normally without glueing this should not happen and if it happens after a short period of time, you normally can change the rubber for a new one at Joola or your shop.
But if that is always happening, it seems the rubber can´t stand your style of playing it.
To get out of trouble, maybe you should change the rubber.
I hope my english is not too bad, long time out of school
Greetz