Without knowing the temperature of your beer it's not clear what the actual problem might be. Take a look at this carbonation chart,
http://www.kegerators.com/carbonation-table.php
plug in the beer temp, go across the row to find the desired carbonation level, the go up the column to find the appropriate CO2 level. If it's lower than your 20 psi then you'll want to shut off the CO2 and start burping the gas out of the keg headspace until you tame that brew...
Cheers!