I read through the previous question and answers asked on here, but it did not help me solve this problem. I recencly replaced 5 GFI Receptacles in my kitchen and bathroom. All were only one white, one black, and one ground wire and I attached them to the line section on the back of the GFI, they work fine and test as "correct".
The one I'm having problems with is in my bathroom. There are 2 white, 2 black, and 1 ground wire and I put them into the new recept. In the same way they were attached to the previous one. A problem that came up was that I keep getting the "open Neutral" answer on my tester. There is a current, but anything I plug in will not work. I tried another recept. thinking that it was the problem, but no. here's what I found. When using the backwire technique, the load neutral will go in and lock tight when I turn the screw, but the other one (Line) doesn't go all the way in, and no matter how tight I make the screw, it comes out. So instead, I looped them each around the appropriate screw, but it still didn't solve the problem. Unfortunately, this is the only outlet in the bathroom and my girlfriend will cut my head off if I don't fix it soon.

You will need to use a voltage tester to find the line neutral, testing between the blacks and ground to find the line hot, and then test across the white in that cable, should be the line neutral.