Tahm Kench grey Hp bar isnt working

Today while playing Tahm Kench, i noticed that my e passive was not working so i couldnt ever give myself the shield. Any reason why this is happening? If it helps my team was against a teemo(jungle sated), illaoi, caitlyn, leona, and leblanc. I really don't understand why it stopped working partway through the game. It was fine at the beginning. Any thoughts?