Are you definitely logged into the proper Google account in the YouTube app? Swipe in from the left to see if the correct Google account is listed there.
I wonder if it's a specific Nexus Player issue. I assume it's fully updated, right? Try wiping the cache partition. Follow the steps here, but select Wipe Cache Partition instead of Wipe Data/Factory Reset: https://support.google.com/androidtv/answer/6121450?hl=en