The AI-controlled terminators do not heal themselves. As a squad leader, you have to keep an eye on their health and order the Apothecary to heal whoever needs it (right side of the wheel). The medikits have a cooldown time and a limited supply, depending on the difficulty. Going through a psygate refills your Apo's medikit supplies.
Of course, in multiplayer or if you play as an Apo in solo Special Missions, Heal is a Skill and thus can be used as many times as needed.
@streumon-f8 thanks for the prompt reply but the order menu still makes it hard to see how/if the order has been given. Nothing seems to be highlighted when attempting to do so, especially in the hectic jumble of battle. Also please pass on to the devs that when creating a confusing but apparently slightly complex order system there needs to be a concise description of how to get full use from it at the least or preferably include such in the tutorial seems obvious to me as smashing multiple walls for a useless relic and a switch that does nothing is possibly less useful in the scheme of things.