The mast will rotate fully, but the fact that it is popping up should be looked at. Check that the connection between the mast base and mast step is not worn to such an extent that mast simply pops up due to the pressure. Can either replace, fit SS Bolts (there are a couple of ways to sort it out) or rebuilt the Al. In light winds and choppy conditons, the mast will thump the stops..nothing wrong with it...
Ask away if you eed more info..would be good to have a local Hobie sailor check it out to help out:)