I recently got really into straight Virginia tobaccos, flakes especially. I have had the same experience. 90% of the time or more, I love it. But every once in awhile, in a clean pipe dedicated to Virginia flakes, with the same tobacco, I get a really lame bowl with no sweetness. I do nothing special and it's a great smoke again another time. It could be cadence, it could be what tea I am drinking, it may be my last meal, maybe it is my frame of mind. I don't know, but I have had it happen very rarely.