This seems to be a general problem. All of the white stacks in the lower left marched through the enemy province indicated by the blue arrow. I wasn't able to snap a very very close unit to the province center to attack and when it got there it just said waiting, so it wasn't recognizing it as an enemy province. At that point I gave it another attack command and it still didn't capture but started counting down from 30m. I assume on the next tic it will capture like in the example above. I think the reason is that the green units are allies. They are allies with both me and the enemy, but I'm guessing the issue is arising because they are allies with the enemy and have units somewhere on that province. This would be consistent with the above example, where that green unit was my ally on my province. It's conceivable, and also consistent with both examples, that it's the other way around though; that it isn't capturing because there are my allies on an enemy province.
update: It did not capture on the next tic but finally did several tics later. There were still green stacks on the province but they were further from the prov nexus. Maybe it has to do with distance (view range?) of allies from the nexus.
