I believe that I have a Bridgwater-Brunel save where this bug occurs repeatedly. I know that B-B saves are not ideal, but this case is simpler than the others, because it only involves a single destination factory served by a single short line. This screenshot shows the context:

Top right: Ironworks (7493,1458) has no coal in storage. Even if it is not producing, my understanding is that coal that has started on its journey towards that factory should continue on its way.
Top left: Chatingpole King Edward VII Dock has 1,447t of coal ready for loading. The chart shows that this situation has persisted for almost a year (in fact, many years).
Middle left: Bulk goods convoy 10110 has departed King Edward VII Dock unladen.
Bottom left: The only other stop on that line (INC Chatingpole Bulk Wagons) is the one serving that Ironworks.
Bottom right: That line has not delivered any goods for three months.
For some of those months, the line was set to wait until 100% load at King Edward VII Dock, but the convoys just sat there, so this setting does not appear to be significant.
The savegame (419MB) is available
here and I believe the screenshot contains all the information needed to locate the offending convoys. Play on / fast-forward from here and watch the convoys fail to load coal. Left to themselves, the next convoy loadings should be at roughly 3:28, 3:40, and 3:50, though you may be able to reduce that by redirecting them to the Dock at game load.
The convoys are all 1 tile length, so the platform length issues reported earlier in this thread should be irrelevant. I added a single-vehicle convoy just to make sure and it made no difference.
As always, if this is not a bug but my mistake, please point out my error! This is a complex simulation and it's very possible that I've missed something.