GAMEPLAY BUG [Relic Shield]

Everytime I go support; I play Thresh. I will buy the item [Relic Shield]. According to the item I should be able to execute minions at a set hp, it's simply not doing this. It's costing my ADC and I to be disorganized and fall behind CS wise. It's very trouble some when in the laning-phase. Riot if you could look into this... It's be great!