Nope...that has little to do with it, since many folks often dump everything into their fermenters.
The only thing that "causes" inconsistant carbonation is a lack of patience, usually. There's a couple other factors if you are priming each individual bottle with priming sugar or carb tabs....Like adding to little or too much table sugar to a bottle, or miss putting carb tabs into a couple bottles....
but with batch priming, MOST of the folks who think they are having inconsistant carbing issues, simply aren't waiting long enough for ALL the bottles to carb.....each bottle becomes it's own individual microcosm, so they all end up having their own timeframe, but it usually is all around the same time...
If you are checking them at three weeks and one is carbed and one isn't really usually only means that you haven't gotten to the point where they all would be carbed...meaning had you lets same not opened them til 6 weeks (if you went out of town) and they all were perfectly carbed, then you wouldn't think or know they weren't carbed at 4 weeks.
If you haven't already, read my blog
Of Patience and Bottle Conditioning. I go into extensive detaul about the carbing/ conditioning process.
How long are you waiting to check on your beers, and at what