Cant get out of combat in arena so cant regen
That would be because there are enemy players around you. I’m assuming you mean the arena in Obsidian Sanctum. Try heading towards the entrance tunnel or anywhere else where no other players are near you.
sometimes i run from one side of the map to the other side and still i do not regenerate. I even have to hide so i can heal myself 3 times just to get enough health to fight someone else. And i play with my friend and he is also a thief so we kill them and he regens and i dont. Happens a lot to me