Background: Summer is upon us. After resuming my cold-brewing regimen, I'm finding that cold-brewing coffee isn't working as well as I seem to remember it did last summer. For those unfamiliar with the process, you take ground coffee and let it sit in cold water from 12 to 24 hours, then strain it. You should end up with a coffee concentrate, to which you add hot water. The main problems are that the coffee is significantly weaker than it was a year ago. I'm using the same proportions, the same volume of water, and, unless I'm misremembering, the same amount of brewing time. The only way I can get a reasonably strong concentrate is by brewing for 12 hours, replacing the grounds with fresh ones, then brewing for another 12 hours. (I haven't tried a straight 24-hour brew yet.) The question: I've used three kinds of ground coffee, and the best results are from pre-ground Chock-full-o-nuts. (A local brewer's beans and Peets beans both produced weak, watery coffee that can't be diluted. Both used fresh beans.) Aside from the obvious (use moar coffee, etc), what can affect the strength of the concentrate? Might the quality of our water have changed? Are there environmental factors to consider? 
Grind     You're using a fine grind? I've always   found grinding it myself to extra-fine   works best for cold-brew. – Bruce   Alderson   and      I pour it through a standard mesh a   few times, then a mesh + cone filter.   The resulting coffee is thick, dark,   and has very low acidity (very nice   over ice with a spot of condensed   milk). – Bruce Alderson   Bean Age     If the beans themselves have been   sitting on the shelf for longer - it   means they have lost a lot of their   fresh, volatile flavours. May not be   the only reason - but it definitely   affects the flavour you'll get. –   Taryn East   Temperature     Is the ambient temperature cooler,   perhaps? It doesn't sound like you   moved, but maybe it's a cooler   beginning of summer, or you're using   more AC? [...] – Jefromi  