[Shan]

My first guess: They are out of contact with their company or battery commander - check by clicking on them and you might see a red message "out of contact" - move them closer to their company commander, preferably in visible range if that´s the problem. This problem occurs frequently if you have an infantry company with a directly attached mortar platoon - the infantry advances and the mortars remain where they are to carry out fire missions. I always detach them in the deployment turn to avoid this problem.