Just finished my second kit second stage and 21 day bottle finish period - a little sed in the bottom but no real carbonation.  Inverting the bottles to check for a bad seal doesn't reveal any leakage....just wait it out or what? Tastes good but flat.

    What temp are you storing them at?  If they are in a somewhat cool spot, try moving them to a warmer room and letting them sit for a week or so.  I had the same problem once and this did the trick.