How long has the beer been in the keg?
The process of carbonation is mostly mechanical. The CO2 has to "laminate" into the beer, otherwise it all comes out at once. Great head, no fizz. The process takes 24 to 48 hours to complete.
Was the beer fully chilled when you carbonated? Warmer temperatures inhibit absorption of CO2.
Finally, make sure you have the right diameter and length of tubing for the pressure you want in the beer. Balancing a draft system can get complex, but you can follow a simple rule of thumb: The smaller the diameter of tubing, the more resistance. You want the resistance of the tubing to be just about equal to the dispense pressure of the beer. An ID of 3/16" is ideal, IMO. It is just about 3 pounds of resistance per foot.
If you want 15 PSI of carbonation (about 2.3 volumes) then you'll need an equal amount of resistance. That comes out to roughly 5 feet of line.