I had this happen to me in vanilla ALL the time.

The best solution is to pick each archer's target individually. Choose your ranged unit, attack the target, then repeat with the rest of them.

I also had the common problem of moving your entire army in formation and it creates a single-line. Found out that you can't move your cav and your inf/ranged at the same time..now i always move my cav in groups seperate from the infantry. Haven't had a single problem since

So basically, if your units aren't listening you need to give the units individual orders.