Use some electric contact cleaner and try to carefully get the knobs to unfreeze maybe, sometimes you may need to take a pair of Needle noses Pliers and slowly try to pull the knobs out, they may be pushed in and stuck, older radios do this, had a grant that I had to do this with and use a bit, I mean a little bit of 3 in 1 oil to loosen the knobs as they were pushed in and they would not come back out.
Also make sure they aren't already pushed in as well. Or vise versa. The electric contact cleaner would be the 1st to try.