maybe they were dead to long ? there is a window where you can res people, after that period of time it won't work anymore, at least thats what i was told
Generally that occurs because the player has logged out. Very common during rage quit attrition pushes where gaining 5 feet up the hill is "successful".
If you redeploy you can't be resed, but then again your body dissapears just after falling, and there is a time limit on how long after a death you can be revived, not sure how long though. But I agree that the scull above their body should be removed if they can't be resed... Also, I have noticed another bug, that you have to hold down the applicator a bit longer then for it to make the full circle to actually make the res stick, I have been on the receiving end of this too, and you see the "do you want to res" option come up for a second, and then it goes away again really quick, so the medic has to apply it again... Rather annoying, since I have the maxed applicator, and having to wait longer then the circle gives like 20-50% longer res times D: