i don't know if this has to do with it, but till some months ago i was doing my purl stitches on the oposite direction and twisting the stitches on the purl rows. so when i got to the knit rows things were tight.
That's exactly what I was doing for almost a year. I finally figured it out and re-trained myself, but it was a pain.
It was okay at first because I started off knitting through the back loop (I didn't know any better) so by wrapping my purl the other direction, it was actually UNtwisting the stitch. But then I stopped knitting through the back loop but didn't realize I'd have to change the way I wrapped the purl as well, so I ended up doing everything in twisted stockinette without realizing it.
As far as the K2tog, I'm still a bit of a tight knitter, so sometimes I'll insert the right-hand needle from the right (sort as if to purl) and tug a little to loosen the stitches up before I try to put the needle in for the K2tog.