I brewed an English Strong Ale back in December that took me a long time to finish up. I had some stuck fermentation problems and went through several steps, including pitching more yeast two times, before it finally finished up. After about 2 weeks in the bottle, I popped one out of curiosity, and it was way overcarbed. It had a thick, foamy head that filled the glass. Another one a few days later was the same way. So I moved them all to the garage where it was nice and cold for a few days, then moved them back in to a stable temp inside for long-term aging. I've popped one from time to time since then to see how it's coming along, and they've been flat every time. I'm now looking back on 2 overcarbed brews and 4 flat ones.
Is the only possible explanation here not thoroughly mixing the priming sugar in? I've never had that problem with any of my other brews.
I'm going to roll them around a bit and move them back to a warmer area in the house and see what happens, but I'm puzzled as to how I ended up with such a wide range of carbonation.