Game goes blank but sound still on - iPhone

[luca0405]

I start the game and the mining and everything is fine. But when I go to the store or the tower then want to go back to the mine, it gives the sound but the picture is blank. I can move the blocks (sounds) but if I try to hit the pause (upper left corner) to go back to the previous screen it doesn't react to anything.

This happens only sometimes, not every time I launch the game.