Another Carbonation question

just finished my first batch, and it's been in the bottle for almost 2 weeks now. I first tried it after a week, but the carbonation level seemed a little low, after another 4-5 days, I tried it again and it was great. had a nice head and many bubbles. Since then, I've opened about 8 bottles total, and 2-3 of them were very flat- almost no noticeable carbonation.
I bottled in 12 oz bottles and 22 (or 24) oz bottles. 2 of the large bottles were flat, and all off the small bottles were well carbonated.

Any ideas of what went wrong???

the only thing i can think of right now is that maybe i didn't mix the DME in enough before bottling (i only had one carboy at the time, so i poured the sugar into the beer and stirred gently)- maybe it just wasn't evenly dispursed.

thanks in advance for any input....

Sounds like uneven distribution to me. Just stir it more often between bottles.

uneven distribution as my esteemed colleague HB 99 pointed out, or caps didn't seal well.