In my campaign as the Turks, most of my Qapukulu are in the 2-3 silver/1 gold chevron experience range, and they never fail to do a charge successfully. They've always been a war-winning unit for me though ever since I got them, I never noticed any charging problems even when they were fresh from training.

In custom battles, though, even when I give them three gold chevrons they sometimes screw up. I don't know what it is - the charge problems only seem to happen in custom battles for me.