Is the tip loose? If so, then you probably don't have the clamp loose enough to get it completely off. If the bolt is backed way out, you might have to pull the clamp open with your fingers to relieve the spring tension.
If the clamp is loose, and the tip is stuck, give it a tap or two with a rubber mallet or your hand (cover the tip with a towel first so that you don't mar the finish) like runner2go suggested. Sometimes they stick in place due to heat and corrosion, but will break loose with a tiny bit of persuasion.