[Sombre]

H1 priests can only raise dominion above a certain level. iirc it's something like their base holy level (x2 if they're at a temple) +1. I'm probably wrong there, but I'll be corrected. It's pretty widely known.

If you absolutely must have dominion in a certain province, using priests preaching is far more effective than raising dominion spread in other ways. Stationing something with domspread there may also do the trick, though it isn't certain to be added to the dominion of that province, so still, preaching is better.