I've been having the same problem - every time I've used sap points, I can't get any units to do the sapping. I've tried skirmishers, infantry, archers, in phalanx, out of phalanx... All I get is the X and the message saying 'Sap Point 0% damage'. I can get units to pick up and put down siege gear fine, just not use the sap points.

Any help?