Slitherine

Hi - first of all, I have recently discovered this game, and after having played the free stuff, I bought every DLC. I really enjoy this game, and there is SO much content, that sometimes I wonder if I ever will have the time to finish all the campaigns...

I keep getting better understanding the mechanics of the game, but one thing puzzles me, and I cannot seem to find a solution.
Sometimes, units of mine (infantry, tanks, artillery) seems to run out of ammo or something like that!? They just cannot attack anymore, all of a sudden. It is not because they are locked in, out of supply, and other units around them seems to function fine. Even when they are at full strength, or I give them a break from the fighting, they still seem unable to attack. They can however still defend themselves. Sometimes, after 5-10 turns, they seem to work again, but I don't know if I could do anything to speed up their repair, so to say...

So, something is happening to these units (lack of ammo or perhaps discouraged), but I don't get any notifications anywhere, so I don't know what to do?
What should I do, and what does it mean, when they will not attack anymore? Should I remove them far away from the frontline, or do I just disband them, or upgrade to another unit?

Your units most likely hit 0 efficiency. It's the battery-icon of the unit panel.
Attacking, defending, getting hit by units with shock values (explosion-icon) like artillery all substracts from the maximum of 10 (=100%) efficiency capacity over time. You should notice that the strength numbers also turn from white to yellow, orange and red color after a while. If the efficiency drops below something like 6, you should really consider giving those unit a break for a turn. It's always useful to combine a rest with a refit to regain (full) strength by the way again. Moving away your battered units from the front-line at first is a good idea before rest & refit. They may not be involved in any action like movement and combat for a single turn to recover efficiency.
Units with more experience (stars) loose slightly less efficiency from simple attacking and defending, but straight substractions from a shock value always fully apply afaik.