
i feel for you buddy, nothing worse than pouring your soul into a brew and then having it not turn out.
Your priming rate and time left to condition seem right so I guess the only other possible causes of flat beer would be
1) the yeast was already dead when you bottled
2) the bottles have been left in a spot that is not warm enought to get bottle-fermentation going
3) your bottles are leaking.
Looking at these in turn;
1 - Excessive heat and excessive alcohol levels (among other things, but they're the most common) will kill yeast. How do you prime? If you're bulk-priming and dissolving your sugar in boiling water this might be the cause (unusual it would kill ALL of the yueast, but stranger things have happened). Is there anything else that would have caused the beer to heat to a point where the poor little yeasties got fried? Or is the ABV% of your beers over 10%?
2 - Where do you store the bottles to condition them? What temperature would it normally be during the day?
3 - Assuming your bottles aren't cracked (which would surely cause bigger problems than flat beer), they may not be sealing properly. Do you bottle in glass or PET? Crown seals, twist tops or swing tops? Do you use a bench capper or hand capper? Do you hear a "psshht" sound when you open the bottles?
If you can provide answers to any of these we may just solve your problem yet.
Chin up,
Merlin