Inconsistent carbonation and flavor

So, I have my 4th batch in primary awaiting some dry hops in the near future. Just bottled my 3rd batch over the weekend.

In other words, I'm a newbie. The only batch that is ready for drinking is the Olde Luddite Strong Ale from NB that I did as my first batch. I've tried a few bottles at various points in the bottle conditioning timeline - 2 weeks, 3 weeks, a month and now we're close to 2 months.

The couple of bottles I had over the weekend were perfectly carbonated and very tasty. The bottle I just popped open has less carbonation than the last couple I had just the other day. And the flavor doesn't quite seem as good either, although that might just be a byproduct of low carbonation. Both the bottles from a couple days ago and the bottle I just popped were put in the fridge at the same time (round about last Wednesday night).

Anyway, here's my theory:

When prepping to bottle, I poured the priming liquid (2/3 cup corn sugar & 2 cups water) into the bottling bucket. Then siphoned the beer into the bucket. I figured the liquid coming in would cause enough of a swirl that I wouldn't need to stir it up at all.

I also had an issue with a leaky spigot on the bottling bucket, so I bottled as quickly as I possibly could to avoid losing any more beer than necessary. With the speed of the bottling, I might have made other mistakes along the way.

So, is the inconsistent carbonation and flavor explained thoroughly by the lack of stirring in the priming liquid? Or are there other possibilities.

NO!!! You always got to boil the corn sugar in water to sanitize it! And then stir the beer GENTLY to get good mixture through out.

Yeah, I boiled it. I didn't just mix the sugar and water and pour it in.

Also, I just popped another one and it is about perfect. The carbonation is great. Taste is much better.

Is it possible that the one bottle just wasn't sufficiently sanitized before it was filled with beer? Maybe just one bad bottle that was contaminated in some way?

I don't stir at all, just let the racked beer swirl in like you did and have never had a problem. The first thing I would look at is the caps and your capping procedure. I use the Oxygen caps always. Occasionally you will run into a bottle or two that just needs a little more time.