Originally Posted by Yooper
Since it's not happening to every bottle, it sounds like an uneven mixing of priming sugar into the beer. If it's not mixed in well, some bottles will have more and some bottles will have less.
Agreed. If you're adding sugar to the bottle and then filling them, make sure it's the same amount (or better yet, considering making priming solution and adding it to the bottling bucket). If you're already adding it to the bottling bucket, try adding the priming solution to the bottling bucket first, then siphoning the beer ontop of it. It will mix better. Also GENTLY stir the beer around with the solution in it, dont splash it around or it'll oxidize.