I made a batch of American Pale ale a few months ago. It clarified in the secondary for one week and a lot of yeast fell out of suspension.
I bottled it and drank one after one 1 week. Completely flat. Then 2 weeks. Still flat but hissed when I opened it. 3 weeks. Still flat and a little hiss. I thought it was done for. Magically, after 5 weeks, every single beer was perfectly carbonated.
So I say, give it at least a month before you start to worry.