Ours does this a lot -sometimes it can be something thats blocking the holes in the spinning "arms". This seems to leave deposits of god knows what on everything (in our case it was a tiny bit of eggshell that had blocked one of the holes).
We find that the stuff on the top shelf,back left corner seems to suffer the most so it helps to not put glasses there as they seem to attract it more than, say, the kids plastic plates.