When you call your droid after it has died, sometimes it has little to no health.
To fix this you can do one of two things:
1.) If you are in Restuss head over to the thieves and attack one; then have your droid attack, and then just finish it off. Your droid will start to heal just like an NPC when it loses combat.
2.) Another quick way is just to duel a friend, have the droid attack, then end the duel. And presto the droid heals back to full health, ready for battle.
I use this all the time in Restuss when the opposing force out numbers us and we go in quickly; we just duel to heal up the droids and run in to kill the overts.
I hope this helps because I always find waiting forever for the droid to heal annoying, but I am just impatient