We have a lot of 2860x's in my environment and they all report without issue. I would suggest checking the IOS version and verifying proper snmp access. Those have been the culprits of most of my issues after upgrading.
So I have checked the IOS versions and the snmp configurations on a stack that does show up and a stack that doesn't, and they are identical in IOS version, and SNMP access.
Good thought, but no dice...