Thresh, relic sheild not proccing minion execute?

sssooo played thresh a few weeks ago as support, and noticed that whenever i went to execute a minion with a relic sheild charge it wouldnt allways proc propperly. i checked the item effect and it says it killed them at or under 240 hp. so watching a minion i tryed doing so and it failed to go off. only time it seemed to work properly was after i got my full passive from threshes e but then thats a bit of a waste on a minion.

might have done things wrong im not sure but if anyone else has noticed this i havent heard. ust thought id post here to be sure if it is a bug it gets fixed.

thank you for reading ^.^