If the warm water soak (30 minutes is usually fine, even for snakes with most of their skin stuck) didn't work, then overnight in a wet bag definitely should have done the trick. Are there several layers of skin stuck in those places, or just one? If its just one, then perhaps you aren't picking at it hard enough? I'm not saying to be rough, but I can't imagine not being able to pick off the skin after the two methods described above.
The stuck skin on the head might come off during the next shed, but stuck tails seem to stay stuck over and over until you get the skin off. If too many layers of skin stay stuck on the tail, it will eventually cut off circulation, the tip of the tail will die, and finally fall off. I think you've got plenty of time to make things right before that happens!
Like I said, 30 minutes in warm water should allow you to pick off the skin on the tail. Its a little harder with the head, since the snake will likely keep its head above water during its soak. As long as it is wet though, the skin should soften enough to get off.
Good luck!
