Is the beer chilled? That will help the CO2 go into solution. Did you check that there's gas in the tank (weigh it) and that it is flowing? (push on the stem on the gas ball valve and see if gas comes out)
Pick up the keg by either end, turn on the gas, and gently rock it back and forth like you're trying to make a ball roll from the top to the bottom and back again. Listen to hear if gas is moving. You should be able to hear it at the tank/regulator, and you should be able to hear some bubbling inside the keg.
With a chilled keg and 30psi, I carbonate my beer this way in about 5 minutes. Let it rest a few days, chilled, vent the pressure, and see how it's going. Adjust if necessary.
I never really liked the "leave the gas on the keg" approach since it takes a long time and can make your tank run dry if there is a leak anywhere. But it does work, so if your keg is totally flat, start the Christmas tree light troubleshooting technique (start at one end and make sure each piece is working properly, starting with the tank of gas).