How to with Multisite?

I can’t find any documentation of how to get caching working on a multi-site (as if multi site isn’t complicated enough). Some of my sites have the setting enabled and I am able to change the settings, but other sites are disabled and display the message “Caching has been disabled on this blog on the Network Admin Sites page.” No matter where I look, I can not find out where this setting is. I didn’t set this setting, nor did I hide this setting so I could never find it again (well, actually that’s very possible).
Please can you point me in the right direction so I can enable caching across all my sites.
They use the structure: demo1.mydomain, demo2.mydomain, demo3.mydomain etc.
Many thanks in advance.