My first step would be to invert all the bottles a few times to get some yeast back into suspension, then store it at warm room temp for at least a week. It could be that your yeast dropped out before it finished conditioning.
Also, if you didn't stir your bottling bucked after adding both your beer and priming sugar, then you may end up with a mix of undercarbed and overcarbed bottles. I wouldn't blindly add sugar to every bottle unless you know all of them are consistantly undercarbed.