I have noticed that servers in my SharePoint 2010 farm are showing statuses like "Installation Required" or "Upgrade Blocked".
It happens when you install the SharePoint patches across the farm but somehow some of the servers are unbale to upgrade during patches installations.
Installation Required : The server still not yet installed with the same CU that already installed in other servers in the farm.
Upgrade Blocked : The server is blocked from upgrade as they are on latest CU than other servers in the Farm. Current Server is installed with latest CU and it will remained blocked until all the other SP servers in the farm are installed with the same version.