Use the relief valve to dump the pressure. Wait, and then dump more as the CO2 comes out of the beer. Repeat.
Now take the out side post off, and pull the dip tube out. Clean both, and pay attention to the out side poppet, it may be clogged with hop particles and spooge.
If it works for a while and stops, repeat the procedure.
How do I know this? Let us just say...