Tap won't pour when fully open, so foamy beer....please help!

Hello,

Not surprisingly, my kegerator is pouring foamy beer because it will only pour if the tap is 1/2 open. If I open it fully, like I know I'm supposed to do, then nothing comes out.

I've disassembled the tap, and all seals appear to be in good shape. Pressure is at 10 psi, temperature 38 degrees. I'm pouring a Kolsch that has been tapped for about a month. Lines are short and on top of the keg.

Also, I'm not sure if this would have an impact, but I didn't use the kegerator for about 9 months before tapping this keg, and it was working fine on the last keg (I cleaned the lines between kegs, of course).

Any help would be greatly appreciated!

Ian