Sometimes my heals just fail and I don't get why

This is a really confusing one for me, I'm sure there is a mechanic somewhere I'm not understanding, but basically, unless I have reach, or the mythic ability for ranged healing, whenever I heal in combat there sometimes the action will just not go off.

You know how you can click a spell and hover over someones portrait and if you can't get to them it will be red? That's not the case here - it can be if I'm standing right next to them, start of the turn, full move and action ready to go. It'll use the spell slot, and if applicable it will walk (within valid move range) next to who I want to heal, but the spell just wont go off, and it eats my action for the round. The next round, I can use the spell again, it will NOT eat a spell slot and it will use the "queued up" spell from last round. Why is this happening?

This is divine magic, there is no failure chance, the spell isn't being lost, or missing a concentration check, the party member im healing is within range, and same thing will happen if already right on top of them.

What am I missing here?