I've never had issues cleaning them. They aren't super easy to clean but I came up with a method that works well for me.

Put it in a pot, add dishsoap, then pour boiling water in it and let it soak for a bit. Seems a bit weird and extreme but it gets majority of the cheese off of it. There's also a technique involving lemon and salt to clean them. Oh and if one has a dishwasher that works very well too, but I know many people don't have them (I practically don't, since the one here doesn't work).