This is was happening before the Isle patch. What I found that bugs them out is when they get sent flying(normally by the Rhino King) and disappear underground. When they resurface their attack animation is stuck with raised fists when they want to attack something. Have you noticed they cannot be knocked down after they start the raised fist thing? If this is your case, try fighting any enemy that doesnt push thralls around and into the ground(ie Rhino King, Red Mother sometimes…etc) and you should not have a problem using them. I play single player so a server restart fixes the thrall behavior.