I've just redoing the missions and I've noticed that you pretty much have to tell your troops to open fire all the time. They just don't seem to notice the enemy within range. I've not noticed them not returning fire though.
You really are missing out on a great side of the game in MP though, but I can understand why some people don't want to try out MP though as you will get your butt kicked for quite some time and patching up the game is over complicated, but hey at least you don't have to spend two days downloading the patches as with CoH