Having trouble opening iBook G4 Case

Putting a new DC In board into my iBook this afternoon, but I'm stuck on opening the bottom case. I've got all screws apart from the Hex screw in the middle of the case, which will not come out at all. Im using a T8 Screwdriver for it, the same one that got all the other hex screws out.
What should I do?

Im using the guide to open it, I just cant get a screw undone, even though i've managed to open the 2 other T8 ones with the same tool.