I had issues with support spells too, at least during the first half of the age. Then I made a thread complaining about it in the Suggestions sections and, probably coincidentally, my success rate improved somewhat. Maybe my WPA caught up. Maybe they did listen to my ranting. Who knows?

I still have some trouble successfully casting support spells on mage provinces (I picked halfling/cleric, which has zero bonuses to WPA) and chained provinces, so I think I'm having to work against their WPA (which sucks). On the other hand, the spells I do manage to get on mages are usually longer-lasting than those I cast on attackers (meaning I'm working with their guilds), though still shorter than self-spells.

In short, the formula is weird and playing cleric has been more or less a waste of an age. Can't wait for it to end.