I have two Coal power plants next to each other at the far end of the city. One has 11 weeks of coal, and the other has none left at all. How do I get coal to be delivered to my power plant?
Why its happening. The car AI in Cities Skylines is quite weird at sometimes, mostly because they find a "better route" which will take them much longer because they think that just because a larger road has a higher speed limit they will get to the place quicker, just to be stuck in traffic. This happens to hearses, fire trucks and all other cars from my knowledge.
How to fix it. For coal plants to maintain enough coal to operate they need access to imports. Putting them near a Highway, Cargo Port, or a Cargo Terminal will help solve this issue. If you are still not getting enough coal imports, try having a small coal industry somewhere in your city, assuming there is coal in the map somewhere. And if worse comes to worse, go wind/solar as it never fails.
Hopefully this helps you with your current and future cities!