you aim with the drone at the person you wanna heal, the cirlce (crosshair) will turn green if you're in the right range and its ready to be fired.
honestly at this point i wouldn't be surprised if it would be a bug, given all the things that have been. but who knows, you might just be doing it wrong too.
It won't work that way on yourself, just hit the key for the "ultimate" to revive yourself. that's different. and when "you're all over" your friends. i.e. too close it won't work. like i said, the proper range will give you the green lights (it shows on your crosshair). it doesn't work the same way it did on wildlands in case that is confusing you. it did me at first.