Every follower is coded to only do X damage or Y healing before stopping. This is to stop people using them to farm. But its really annoy when just doing normal quests as they will stop and just stand there during a fight. Sometimes, this 'limit' does not reset between fights (log off and on fixes this)
They will also just 'stay' there if they get near zero hp but that's to be expected.
So they are working as designed, just a very annoy design