Thresh Face of the Mountain not procing

I played 2 games with a friend last night. The first was Miss Fortune and Thresh bot lane. The only way Thresh could proc his Targon's Brace or Face of the Mountain was to actually last hit the minion when it was under Thresh's attack damage. For instance, if the minion had 200 health and Thresh attacked it the minion would not die/heal/grant gold. But if the minion was 20 health and Thresh attacked it, it worked as normal, die/heal/grant gold. This happened after he upgraded Targon's to Face also. It happened the entire game. We tested it multiple times, watching minion health, making sure proc was available etc and it did not work properly the entire game. We Qued again and went Nautalis Miss Fortune bot to test a different champ with Miss fortune to see if it might be an interaction with that champ but it worked as intended. One other note. We both had the Bandit mastery.