Recently, one of the ales I brewed and bottles was a bit flat even after sitting in the bottles for two weeks. The flavor was great (I have already drunk/shared most of it) and the alcohol level was adequate. It was somewhat carbonated, so it wasn’t like truly blechy flat beer. These was still some fizzle on your tongue and a slight head when poured into a glass. I added 2.5 oz. of priming sugar before bottling for 3 gallons (30 beers). The temperature in the garage where the bottles rested was not too cold, so fermentation should not have been prohibited due to that.
In consultation with Kal at Oak Barrel Brew Supply in Berkeley, I think that I was too fastidious about avoiding the yeast at the bottom of the carboy when I was siphoning the beer into the bottling bucket. I like a nice clear beer with no residue at the bottom of the bottles. It looks more professional. But perhaps there was not enough yeast still floating around in the brew. Most of it must have settled out and so not enough was there to get a good carbonation going.
In the future, I will be sure to allow some of the yeast from the bottom of the carboy to be siphoned up into the brew before bottling. It is better to have a little yeast residue at the bottom of the bottles than inadequate carbonation.