There are several reasons why this can happen.

One possible reason is that a truck gets stuck somewhere on the road.
If this happens, restarting the game reloads all trucks and usually fixes the issue. 

Another situation is shown in the example screenshot:

Sometimes trucks can get into a traffic jam.
In this case, close the game with a force stop and open it again. You may need to do this more than once.

If you see only some of your trucks on the road, the others may be stuck somewhere outside the screen.
Restarting the game should make all trucks load correctly and appear again.