RTW Vanilla Wall-related bug: Help?

I've noticed an insanely frustrating bug with the way units will -- or more accurately, won't -- mount walls during a battle. For whatever reason, on very frequent occasions I will order a unit to take a place on a stone wall and will either (a) not move at all, or (b) will move to a spot near the nearest tower and proceed to...do nothing.

I know this is an RTW bug and not an EB-specific one, but does anybody know a way around it, or at least when and why it occurs?

I bring it up because I just lost a rather important seige battle which I would've won (barely) except my single reserve infantry unit stood stupidly by while a unit of Makedonian slingers took control of the city gate...and then got trampled by the Makedonian cavalry which had been waiting outside the walls. I was literally sitting there yelling "DO SOMETHING!" at the monitor....