Brewed my first batch (Beer Simply Beer IPA kit) and bottled it about 3 weeks ago. Tried a bottle after 2 weeks and it was flat so I waited another week and tried a few more. One was very good but another was still flat and yet another was way overcarbonated and undrinkable. I boiled the priming sugar from the kit and let it cool before putting in the bottling bucket. I then racked on top of the priming sugar but did not stir. Not sure why I'm getting such inconsistency. Ideas?