Many factors can cause this. Simple way I combat it is a 5/16 (I know, not ideal but its what I had left after making my kegerator) tubing into a 1-2 liter bottle with enough water so that the end of the tube is submerged. Put the other end into the bung in your carboy. I've never had an issue with it getting stuck. Once it slows down you can put the airlock on.