Green check marks disappear in "Drive by Route" screen.

Can anyone tell me what causes the green check marks indicating a successful completion in the "Drive by Route" screen to disappear?
This happened in RW2 as well as RW3. It has happened on several of my routes several times for no apparent reason.
Thanks
This happened in RW2 as well as RW3. It has happened on several of my routes several times for no apparent reason.
Thanks