Sheen/Tri-force not working on ezreal?

I just played a game of Ezreal and I bought sheen and noticed that it was not proc-ing when I was near enemy champion or minions. When I was pretty far away from everything, I could see the buff show up after casting a spell but if I was around anything, there would be no buff icon and no extra damage after casting a spell. I then bought tri-force and the issue remained the same.