One thing that affects conditioning times, in my experience, is how long the brew sat in secondary fermenter, how high a gravity the brew is, or if it was lagered or otherwise cold conditioned. Basically, the longer it has been sitting after active fermentation has stopped, the more alcohol in the beer, and the colder the fermenter has been stored, the longer it will take for the yeast to re-activate and consume the priming sugar. 2 weeks works fine for a regular gravity ale that has only been in secondary for a few weeks, but for a strong brew that has been aging for a month or two, it will take quite a while. If you are dealing with an extreeme case, like a barley wine that has been aging for a long time, you might want to pitch a bit of powdered yeast at bottling to insure that there are enough active yeasts to get the job done.
O, guid ale comes, and guid ale goes,
Guid ale gars me sell my hose,
Sell my hose, and pawn my shoon -
Guid ale keeps my heart aboon! -- Robert Burns