My buddy was too lazy to look online for solutions, and actually fix his rock band guitar - so I stole it, the game, his other guitar, and his mic. It wouldn't register down strums, and as you know, up strumming an entire song is just no fun.
I have to give credit to Ben Sones from Quarter To Three. His fix was the inspiration to mine. Plus his seems more reliable.
Step 1: Open It Up!
Step 2: Finding the Contact!
The strummer works by leveraging the other side to push contacts together, which then allow signal to be sent to the XBOX. The problem was that my contacts weren't contacting eachother.
Now remember, the contacts for the down strum are up, and vice versa.
Step 3: Fixing the Contact!
Some will say to play around with the tension screw. I say why do that when you can be lazy.
What I did was put in a spacer behind the upper contact, and then tried down strumming. You'll see the corner of my gas bill acting as a spacer in the picture below. I found it to be a bit thick - the strumming was too sensitive - but it worked!
Now obviously an entire gas would be ridiculously big to fit inside the guitar, so I tore off a corner of the envelope. I found that a little piece, folded over (to equal two sheets), did the job perfectly. I didn't worry about it coming out again since the contact's pressure against the mounting nub behind it was pretty strong. If it comes out again, I'll just open it back up. I played a song with this fix, and it works great.
Please let me know if this works for you too!