I've made about 11 all grain beers now and I have kegged most of those. They all have a good, foamy head that usually leaves some lacing until the end. The exception is any beer I've made with dark malts. A stout, an attempted Smithwick's clone and an English brown using chocolate malt. These have a thin, fizzy head like a can of pop that disappears in seconds. What could be the cause of this? I carbonate everything at about 12 psi, and while it's fine for my other beers it seems to be problematic for these ones.