Tenants didn't pay full rent.

[Mae Furman]

My husband and I own a multifamily home where we live in half and rent out half. 

I typically receive a money order from them for rent. I received the money order for 300 shy of their monthly payment. I reached out and said that they were short on rent and needed to pay the full amount. They said that there were two payments in the envelope, a money order and a check. I only saw a money order but now I am worried I over looked a check. I don't see how that would happen but I guess it wouldn't be impossible. 

So my question is now what?