Marketing Cloud Content Build Reference Block not visible when sent

I have an Email in Marketing Cloud that we’ve added a Reference Content Block to. It displays great in Edit Content mode but as soon as we "preview and test" or ultimately send test emails all we see is:

%%=ContentBlockbyID("XXXXX")=%%

This is obviously not working, we’ve removed the block and tried other referenced content blocks that were working but none of them seem to work now.

Any thoughts? We were literalling this (hold hands so close together but not touching) close to going live with marketing cloud 🙁