I've noticed in a couple of my games now that after building a coal generator and setting it to a high priority, even though I have plenty of coal in storage, it can take up to half a day or longer before Duplicants will deliver coal to it. They don't keep it full of coal either, but let it run out, then it sits idle for another half a day or longer.
Steps to Reproduce
Build a coal generator while having coal in storage. Set it to a higher than 5 priority. Even at a 5 priority they should attempt to keep it going all the time...
Status: Known Issue
We are aware of this issue and have it logged internally but either we do not have a fix ready yet or we are not planning on addressing the issue in the immediate future.