bottle size and uneven carbonation

I've had problems getting a batch to carbonate. It's a moderately strong brown ale (a little over 6%). So far it seems that 12 oz bottles and 11 oz fliptops have finished carbonating. My 22 oz bottles and 16 oz fliptops have all been flat - you can see a few bubbles, a hard pour results in no head, and it would be a lot better with a light carbonation. It seems a little strange - any idea what might have caused this?

For the past couple weeks I've tried keeping the bottles warmer, and flipping them over to try to keep yeast in suspension (they were in the low to mid 60s before, now I manage more like 65-75 depending on time of day). It doesn't seem to be helping - do you think the big bottles are salvageable with more time? (I want those bottles back for more homebrew, especially the fliptops!)

different amounts of headspace will cause carbonation variances, especially comparing 12 oz to a 22 oz bottle.

but I wouldn't think they'd be completely flat.

do you even get a *hiss* when you pop the cap? if not, it sounds like the caps may not have sealed tightly.

No hiss - get pretty much nothing when I open them. There's a combination of crown caps and fliptops, and I've tried 4 or 5, so I don't think it's a bad seal.

I just read "a primer on priming" and figure that I slightly lowballed the priming sugar by a bit, but I should still have a solid 2 volumes of CO2. And it doesn't explain the unevenness.

So do larger bottles require proportionately larger headspace? I'm stumped. I'll give it a couple more weeks, and if that doesn't work I might have to drink it myself (that'll teach me a lesson!).

Did you gently stir the beer in the bottling bucket several times during bottling? This helps keep the priming sugar you just added evenly distributed.

Were the larger bottles at the end of the bottling session?

Moonpile had the same thought I did. Personally, I tend to bottle all my big bottles first, then head to the small ones. I also add my priming solution to the bottling bucket first, then rack onto it. If there were a case of the solution not getting stirred enough, it's plausible that if the bottles that got filled later might have less sugar.

I racked onto my priming solution, and bottled the big bottles first, but I did not stir. So it would be more likely that the big bottles had more sugar. That's a good catch, though - there could have been something else weird going on that kept the early bottles from getting as much. In the future I'll give it a stir.

Another thought, do the caps have to be totally dry before you use them? That would affect both caps and fliptop gaskets, and would affect the bigger bottles more since I bottled them first. On the other hand, for the last few bottles I needed additional caps - I dunked them in sanitizer and used them almost immediately, and they turned out fine...

Thanks for the suggestions. Wish I could put together a better diagnosis of what happened, to avoid letting it happen again, but maybe I'll chalk this one up to bad luck.