Sounds as if there is friction within the mechanism somewhere. Have you tried to fast forward the tape, then rewind it fully ? This may loosen things up a little.
Another possibility is that the tape itself has issues and is sticking within the cassette - have you tried with alternative tapes ?
To which particular model of Camcorder does this query refer, and what is the exact problem, please ?
Or is this perhaps some other item, and your query has been inadvertently posted in the Camcorder forum ?
most electronics / small appliance stores carry universal knobs or some can order a matching set --or do an online search either under make and model or for universal knobs
