Assuming that you primed with the correct amount of sugar, the only answer is to wait.
A typical gravity beer takes about three weeks to carb up when stored at 70 degrees. If temps are cooler or gravity is higher, it can take longer - sometimes, much longer.
Note that once this timeframe is met, you'll get best results by putting the beer in the fridge for at least a couple of days (if not longer) prior to opening it (more CO2 dissolved in the beer due to temp = better carbonation and more head retention).
The beer WILL carb, but you have to be patient.