I feel the same. I started playing Call of Duty Black Ops. I picked it up and got a feel for it really quick. I ended up with a 1.61 KD. The next game that I picked up was MW3 and the same thing, I picked it up quick and ended up with a 2.00 KD even. However, in BO2, I suck. I don't know why I suck I just do. It's really frustrating to me. Perhaps I'm just not familiar with the levels yet. Right now I'm a 1.31 KD. Every time I think I have 'got it' by going a normal 15-5 or something, the next game I turn around and I'm at 10-12. It doesn't make any sense to me. All I have been using is an AR, so perhaps I should start using SMGs... Something needs to change.
I can't complain about lag or spawns really, they have done a good job.