Defibrillator not charging - can't revive for 100% health

Pretty much the title. I saw dozens of short videos online how to charge the defibs. Pick support, hold fire button, wait for a clunk sound, lights will appear on them...

And this is where it goes wrong. I hold the rev for 3 seconds, no lights appear and I get 50 points / see my squadmates at half health. 

I changed my controls mapping, I reinstalled the game, I swapped loadouts, switched guns, gadgets, gadget slots. I tested it out on several game modes. 

Nothing! It's agonizing as I always want to rev for 100% and seeing how such a basic function doesn't work is frustrating me into skipping playing medic at all.

Please look into it!