More than likely this is a poppet issue. I had this problem too, and replaced it with some sort of universal poppet that my LHBS sold and now it works like a charm.
Primary: DunkelWiezen, Imperial Brown Ale
Secondary: Barleywine, Holiday Mead, Munich Helles
On Tap: ESB, Irish Red, Nelson Sauvignon Pale Ale,Coffee Imperial Stout on whiskey soaked oak chips
Bottles:Barleywine, Oatmeal Stout, Irish Red, Clone's Amber Ale