Question: I just bought Left for dead 2 in the recent Steam sale and thought I'd try single player first. However trying to play the first campaign I always find my AI team mates stop defending themselves or attacking zombies at the start of the 2nd level. Which makes it seriously hard to keep them alive.

Has anyone else experienced this and know a solution?

Answer: 
Do this

1. On your Steam Library, right click on L4D2 and click on Properties
2. Go to the Local Files Tab, and then click on Verify Integrity of the Game files
3. Now you just wait for it and if something is wrong it should fix it and redownload the corrupted files.
The AI should shoot and defend along with you. This just happened with the latest patch. Do not believe they just hang out, or this is not normal. I have been playing this game for as long as it have been out, and this just started happening.