I just finished hooking up my Aimtrak and it works great... with one exception: I can't get it to emulate the "middle mouse button."

Specifically, the cursor tracks fine and the trigger/red buttons can both successfully act as a left or right mouse button. However, if I set any of the buttons to output "middle", it does nothing. I've confirmed this with a few programs, including Dolphin and a handy little program called Keyboard Test.
I was hoping one of you may have had the same problem and know the solution. If not, i'm going to have to be more selective about the games I put on my machine...

Thanks in advance!
Some additional info you may need:
Windows 7 64 bit
Gun is in mouse emulation mode
Firmware version 9.15, utility V2.0