I go through this regularly. I'll be smoking a favorite like Balkan Sasieni, and all of a sudden, it's just air. I stick to Virginias for a few months, and try again, and the flavor comes back. It's very frustrating, but at least I know I'm not the only one. Alternating a bowl of virginia or burley between english blends seems to help stretch out the "flavorful" periods, at least so far.