What's the yeast strain? And how long as it been? Some English yeasts (particularly my favorite one) are top cropping yeasts, and even when the yeast have mostly flocculated, sometimes the krausen will hang around for a while. I've had a couple times where the beer has dropped completely clear after 4 or 5 weeks in primary (with FG reached after the first week or so), and there's still krausen hanging around, at which point I've just racked out from under it.