4K TV Resolution Setup - DirecTV

I have two Samsung 4K Directv Ready TVs, one in the Office and one in the Family Room. The Office TV is connected to an HR54-500 receiver, while the TV in the Family Room is connected to a C61K-700 Mini. Setting up TV resolutions on the Family Room unit with the Mini allows me to select 4K. However, on the TV resolutions setup in the Office with the HR54, there is no 4k option - the highest resolution shown is 1080p. Why is this? Since both my TVs are 4K capable, as is the HR54, shouldn't I be able to get 4K resolution on the unit connected to the HR54? Thanks in advance for any thoughts.