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) 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?