Since a while now, NPCs reset their stamina to full when they go out of combat. This is causing issues when trying to capture the tougher thralls out there, as on heavily loaded servers NPCs will glitch out of combat regularly, making it virtually impossible to capture them. Please consider rolling back this change.
It was an intentional change since people were cheesing them from rooftops with blunted arrows where the thralls couldn’t act.
All thralls can be captured fairly easily though… if you need more information on how, you could check out this thread: