BrianH1988 opened the ticket and wrote:

The supply route in question has the following setup:
MPS Oil Tanker (sea) with the following route:

1. X Oilfield (water tile about five tiles away from land)
2. X Docks (connected to road)
3. X Ship Depot (a few tiles away from the docks)
   Witcombe Oil Tanker (road)
4. X Truck Stop (in cell directly connected to and directly next to X Docks)
5. Y Truck Stop (down road directly connected to X location, in cell directly connected to and directly next to Oil Refinery)
6. X Vehicle Depot (a few tiles away from X Docks and X Truck stop, connected to road that Witcombe Oil Tanker travels on for destinations)

Everything is set up properly so far as I can tell. The ship will depart to the oilfield, fill up as much oil as it can carry, and then will go to the dock and then move on without unloading, even when I have a vehicle registered at that stop. I let the simulation run for quite a while before it finally started working- the route for some reason was taking forever to register, and when I manually created a fleet of vehicles and manually gave them orders to do the same route, none of them would actually transport anything after confirming the route, even though there was still more oil left to be transported (it would pile up into the millions of gallons before it just started ignoring again). Duplicate orders on the original vehicle worked fine, but multiple vehicles going to that same dock to load and then unload at the refinery did not. I went so far as to make two extra docks connected in the same manner, each getting one boat to their location, and each going to a different oilfield further away and they exhibited the exact same problem. The other routes were connected in the exact same manner about 10 to 15 tiles apart on the same coast and had two vehicles coming from the same road depot. None of this worked after several in-game years of letting the route figure things out. The one vehicle and one boat that I had gotten working took maybe a few in-game months before it figured out what to do. I had tried this once before on a prior map on an inland body of water with the exact same setup, same problem.

Reported version: 1.6.1-RC1
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/6480