Well sixth edition has been out for quite some time now. The new edition is very long in terms of all the rules, many of which are new or modified, and it's much more complex overall when compared to the past two previous editions. I see people make fairly significant mistakes all the time or not using a new rule that could be useful for them. I think this is pretty much always the case when a new edition is released but it seems this time it's taking a bit longer for most of us to completely digest everything.
If you consider yourself a competitive gamer then you must know all the rules inside out. I have started to reread the rulebook and have discovered a lot of things I was either playing wrong (for whatever reason) or rules I was not aware existed. As is often the case we sometimes slip up and play some of the rules according to previous edition. There are also lots of new FAQs out now that have been modified several times and then there are two new codices... It's a lot to keep up with but like I said it's very importsnt to fully comprehend all the rules.
If you want to play competively and win I highly recommend picking up your rulebook and then read it again. I think if you do it will tighten your game and I bet you'll do a lot better.