Aura of Protection doesn't affect Concentration Saving Throws

The Paladin's Aura of Protection should affect Concentration rolls since they're Constitution Saving Throws, but right now I get no bonus:

My Cleric rolls with a +5 modifier despite the Paladin with Charisma 18 next to him.

But this seems to be a broader problem with Aura of Protection: sometimes it works, sometimes it doesn't, almost at random.