You may have an issue with there being enough yeast left to carbonate the beer. Either gently stir up the yeast cake a little, or at least take some out, or add some ale yeast from a fresh packet at bottling time. You would only need about a quarter to third a packet.
I have a lager that's been in the bottle for a few months now that's flat. I thought I stirred the yeast up, but it was either already dead or at I didn't get enough. It is getting me to the point where I want to buy kegs so I can at last drink this beer. I had one bottle that was carbonated and it was great. I've also had a bottle that was too sweet. I need to pay more attention to my bottling procedure next time I bottle.