Tait's biggest weakness is his lack of pace. In the past, his other talents have compensated for that.
When he plays like he did against Hearts in the cup game, his lack of pace just amplifies his mistakes. (Unlike, say Dunne, who has the pace that can get him out of trouble when he screws up).
I don't know if he was injured or sick for that Hearts game, but it was one of the poorest performances I have seen from him. The fact that we won the next 3 on the bounce when he was dropped meant there was less chance of him making a come back.