2007 Ford Focus passenger side mirror replacement

2007 Ford Focus

Hi,
I'm trying to replace the passenger side mirror of my 2007 Ford Focus. I have successfully detached the old mirror but can't get the new one back on. The problem I'm running into is that I can't get the new mirror to lie flush in the groove and the hex screw that goes from inside the car door, into the center of the mirror, won't catch the thread. I just keep turning and turning the screw and it remains in place.
If I line up the mirror without using the screw, I still can't get the mirror to lie flush. And if I try screwing the screw directly into the mirror, without going through the car door, it goes in successfully so I know the thread is still good.
Just to clarify, there is a Torx screw that fastens through the car door and into the corner of the mirror, but that is not the screw that is giving me problems. I have tried putting that screw in first and then attempting the center hex screw. I've also tried saving the Torx screw for last, but still the hex screw won't go.
Any ideas? Ever heard of this problem before? I've tried removing the padding of the mirror to see if it's that thickness that is causing problems. But that didn't work either.