Question about GREAT STUFF Foam background?

[BettaShawn]

I've sprayed, siliconed, and dirted a GREAT STUFF background in my 10g tank. Its been sitting for a good week or two now. When I was done dirting it, everything was all big and swelled like I expected. Now, after maybe two weeks, its like the foam has shrunk. All my walls are like 2" less then what they were, the holes I had made for plants are now just crevices. Is this normal? Is it suppose to shrink down after a few weeks? My room is ventilated and properly heated/cooled.

*EDIT* everything is a lot softer now too. I don't know if I should continue with the tank or just go the normal route.

[DonLisk]

I have hear of this occurring before and not sure the cause. Temps could have something to do with it.
I have seen mine separate slightly from the sides but nothing drastic.