Black Ops help ! Please!?

Well, Im actually pretty good, maybe not soo good. I call in about 1 Chopper Gunner every two days. But, Im very inconsistent. Meaning, I get a Chopper Gunner 1 game, then the next game, I keep Calling in Spy Planes, Help please. Also, How come sometimes you can't join your friends game, The " Join Session In Progress " option is just not there sometimes. Should I start playing Ground War or Demolition ? Because they are longer than team deathmatch ?