I have a stout that is set at 11 PSI and fully carbed. The first poor come out with a nice head of Foam then after that the beer comes out slow and creates no foam. I have just under 5 feet of hose line to the tap.
How long has the beer been on gas?
I found that my beer will do what you describe after it has been on the gas for around 5-7 days then by day 10 all pours have a nice head and carbonation.
At what temp do you keep it? Check your poppet valves in your kegs, too. You may have a hop particle stuck in there.