Relationship not seeing fields

[Cat8]

I'm trying to make a relationship from one file to another - as you do! I have other relationships set up from the file I'm trying to make the relationship from. In "Define Relationships" I chose the file I wanted to link to and when the table came up, it couldn't see the fields from that file so I can't make a link.

What could be causing it to not see the fields? How do I fix this??