I'm assuming you're using a 3-piece type airlock and the clear vinyl tubing? Try heating up the end of that tubing in hot water and see if it will fit over that piece in the center of the airlock.
Also, you might want to do with that airlock is cut off the little plastic nipple at the end of it on the side you push down past the grommet. During high krauzen, it will clog very easily.
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