Not receiving chests?

Hey guys,

I've been playing with a friend all day today, and he received about four S rankings and I received two. Throughout this time, I haven't received a single hextech chest or whatever they're called. On my account page, it says that I have four available. He even received one when I scored an S, but I didn't. Why am I not receiving any chests? Is this some sort of bug?

Thanks for the help.