I still get some flat bottles

I made an IPA clone a few months back, that I let sit for 7 weeks now in the bottle. After 3 weeks it was still mostly flat, at 5 weeks I started getting some carbonation. I opened a 22oz bottle the other day and it was great, with fine bubbles and a smooth head. Then I opened a 12oz last night and it was mostly flat. I drank that one (it wasn't undrinkable:cross and went straight for another. This one was fine.

When I bottle, I always just go up and down the line within the six pack, and these last two bottles were consecutive bottles, so even if I didn't mix the bottling sugar in well, these would be from the same spot. I pour the boiled sugar in to the bucket, then siphon the beer in and let it swirl.

The only thing I can think of is I'm not getting a good seal, but that's never been a problem before. Any other ideas?

Bottle seal is the only thing I can think of. Your procedure sounds fine.

+1.

What sort of capper are you using?

A double wing type, two different ones actually. I wonder though, because the bad bottle wasn't completely without pressure. I guess it could be a bad cap too.

Just wondering if all of the bottles are the same, or if your using a couple of different varieties of bottles and maybe that could have something to do with it

When I bottled, I would have a few bottles that didn't seal correctly due to a smaller lip.