Did you properly mix your priming sugar? I have had that problem in the past, and it was because it was not properly mixed. Now I add my priming sugar to some water, boil it, cool it, then add to bottling bucket midway through transfer. Since I started doing that, I have had a pretty consistent carbonation.
AHA Member, Mississippi Brewer's Guild, Hattiesburg Beer Club, Raise Your Pints, and Outlaw Homebrewer