CB750F3 rebuilt. Unable to find 5th gear while riding

I shift and there's nothing there.
Doesn't clunk, grind or make any sound. Nothing. Just like when you go looking for 6th

Shift drum, forks, and dogs in excellent shape during inspection/rebuild.
Birdcage correct, and I can shift through all the gears, and even find 5th when the bike is on the center stand, and i'm gently rotating the rear wheel.
Took shifter cover off last weekend to verify everything was in its correct place. All good. Test shifted through the gears with the cover off as well. Again...worked fine with some wheel turning. I can provide other pictures if they'd be helpful.
Thanks for your input!
