Message category missing from Filter dropdown

I’m trying to export form data for clients, but I’m finding that some of them, in the Inbound Message screen, will not show me all of their forms in the Filter dropdown.

I need the data in separate spreadsheets for each form. For instance, where I have two forms (Newsletter Signups & Contact Form), the options I have in some cases are :

View all channels
Contact Form 7
Newsletter Signup

…but the Contact Form is missing, even though it looks like there are 385 submissions through that form. Why is this happening, and only with some sites and not others? Also, how do I fix it?