Relic Shiled

This has been happening to be for the past 4-5 days. Every time i play Thresh and get relic shield, the passive won't activate UNLESS I am the last person to hit the minion and even then, it only works less than 50% of the time. Relic shield should allow the support to execute a minion below 200 hp and return some health to the adc. Well, for some reason, it doesn't do that. Like i said, it happens less than half of the time and it doesn't execute minions below 200 hp. I don't know if its only on thresh or not but yeah, its kind of annoying :/.