What is the temperature? When you serve it, how much foam do you get? If you are getting quite a bit, then it is probably carbed but the CO2 is getting knocked out of solution when serving. You need to balance your line length for your CO2 pressure and temperature.
You could try knocking the pressure down as low as you can get to push the beer, venting the pressure off of the keg, and serving to see if it is carbonated. This would be a temporary fix, but it would tell you whether the beer is carbonated (and perhaps over-carbonated).
If you are getting no foam, then I am stumped.