Be careful not to apply too much force. The steel plug and the aluminum cylinder head can bind up, and it sounds like this has happened to your head. Try tightening up the plug a bit before you try to back it out. This will break some of the crud off the threads and force it forward. Tighten it up no more than 1/8 turn, then begin to back it out. If you begin to feel a high resistance, then turn the plug to tighten it. Then back it out until heavy resistance is felt, and turn it in the opposite direction again. Each time you back it out you should be making more progress. Repeat this until you finally get the plug out. Take your time. Applying excessive force with the breaker bar will snap the plug off, and then you will have to drill it out and have a helicoil installed to repair the threads.
