This is the second scenario from the campagin "Custom Mongolian Campaign". When I played it, any unit ( player or enemy ) over the bridge would disrupt the destruction of the bridge. So I made sure any troops on the bridges must be removed first. This way the problem is debugged.
Update : you can now see a bridge being made, however as you can not thin the line of enemies before attacking your home base, the scenario is now very hard to beat.