Would you happen to know the strain? I think Sativas and sativa-heavy hybrids may take more than 8 weeks to finish. Also, i've never heard definitively but I often hear that a plant has been flowering for xx amount of time. I wonder - does everyone start the "flowering" time from the day they switch to 12/12 or the week or two after that when the plant actually starts flowering?
One other thing - I think the trichome color/opacity is the best way to determine when flowering is complete. Pistils are unreliable since they often turn red early or do not turn color at all.
Having said all that - it is hard to believe that even if you harvested a month early - you barely catch a buzz from smoking it.