Thunderlord's doesn't appear to proc on Cass

I hadn't played her since her rework and on some ARAM games I've had my mastery page with Thunderlord's but despite people standing in my binding after I hit them with other abilities it did not fire. I thought maybe it was on cooldown or that I had missed it the first time but it happened repeatedly. Am I seeing things or can anyone confirm this? I'm asking before bug reporting.