Which mountain bike shoes? El Cheapo's or something decent? Some mountain shoes are garbage for anything longer than a 10 mile ride. I know I've owned a few.

Something like a Sidi Dominator 5 or higher quality with make a big difference. If you are using quality shoes then maybe it's cleat location?

Just my rambling thoughts.

Ps.. if I couldn't wear moutain shoes/spd's i'd be wearing road/spd sl. Never going back to platforms.