I was doing very well with one of my 2v2 partners originally doing a Roach + Marine rush. Very powerful, tends to annihilate one opponent, and then easily fend off the reinforcements / counterattack by the other opponent. However, as we started playing against Diamond and Platinum teams, our super-strategy stopped working. Why is this?
Simply put - scouting and preparedness. Good opponents know how to interpret what they see in your base, even if it's just gas and when your buildings get produced. The trick is to do this yourself, and counter what they are doing.
For example, we lost a game recently to a Zealot - Marine all-in, which was obviously directed at me, the Zerg player, since I can't wall off my base and shoot the attacking units. I scouted it, but didn't build Spine Crawlers to fend off the massive push. Even with my ally matching the Marine numbers of the attackers, I couldn't take on the Zealots without Roaches or Spines, and lost (despite my banelings).
This situation is similar to what cannon-rushing opponents sometimes find as they climb the ladder - eventually their cheese stops working, and they have to abandon the strategy and start learning new ones.
Morale: Brush up on other races' builds, not just your own, and know what's coming, and when.