Hi
I have not encountered this exact scenario before, but I think it could be a server glitch. There are times when servers respond with delay and cause resources allocate incorrectly.
There is also a known bug which a delivery truck believes there is no where to return to and leaves the city. Therefore, it is best to check on that when trucks do not pick up things, and if this is the case, you may want to demolish the truck parking lot and re-plop them.