Not getting chests

So far i have achieved an S- or higher for the first time on three champs and I have not gotten a chest as a reward. I was wondering if this was a bug or if I am mistaken in this. In my Collection screen, it does not indicate that I have gotten a chest with the champs yet they still say that I have achieved an S- or higher with them. Was hoping you could help me out. Thank you.

P.S. Champs so far with a S- or high but no chest: Graves, Kayn, Teemo