What model Ibanez do you have? Is it one of the strat-type? Are the knobs similar to strat knobs (with a flange and numbers on them)? If so, they are sometimes hard to get off - worst case scenario, you can wedge a small screwdriver under the bottom of the knob and pry up as you are turning the knob and it will eventually pop off. However, in doing this, you risk breaking the knob or scratching up the body/pickguard - though if you are careful, it shouldn't be a problem...