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: 
In my experience, that's just how they are. Just ignore them as much as you can (impossible to ignore completely), and rescue them when necessary. 'least, that's how I usually treat it.
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.