This only happens very occasionally, but it's always really annoying! Basically you clear one level and the black screen with spinning star in the bottom corner appears and the holding music starts and then it stays like that and the next level never starts. It just happened to me going from (I think) level 26 to 27. The music keeps playing and the star keeps spinning, but you just never get to the next level. I've left it like this for half an hour.