Actually, the normal reason for Tilting Bridge not working (assuming that it's functional at all at that moment) is due to the truck that went across immediately before yours. If a non-safari vehicle goes across, the bridge does not tilt. It also automatically does not tilt for the next vehicle that goes across the bridge. However, if that second vehicle was a safari truck it will tilt for the following vehicle (assuming that it, too, is a safari truck).
The same sequence of events will happen if a safari truck goes across the bridge too fast (more than 4 mph).
There's a way to tell if the bridge will tilt or not. There's an orange light at the far-left end of the bridge. It is lit when the truck comes around the corner approaching the bridge. If the light turns off as the truck is about to drive onto the bridge, the tilting effect will happen. If,while the truck is on the bridge, it starts blinking that is a signal to the driver that they are going too fast and risk the bridge not tilting. If the light comes on solidly or does not turn off as the truck approaches the bridge, it will not tilt.
Thamks for the information--I`ll watch out for the clues next time we go!