OK, so I had this problem too. I then realised that all you need to do it click the downed player once and STAY next to their body. The natural instinct is to hold down the mouse when you are trying to revive them like in a shooter but that’s not the case here.
Just click the player’s body which goes highlighted yellow the once and stand next to them while they are reviving.
If you move away, the revive action will stop so stay right next to them.