I've been brewing for a little while now and the first batch was OK and the next 4 were great then the 5th and 6th batches were flat even after 6-8 weeks in bottles there was some carbonation but not nearly enough about the only thing I can think of that I changed was the amount of time in the secondary the 5th batch was about 6 weeks and the 6th ( a lager) was about 4 months. Is the extended time killing my yeast or do you have any ideas on what else could be going on?