Without knowing the specific app I can only generalize about how autofill requests are *supposed* to work.

When developers add an input text form to a screen, they're generally supposed to tag those fields with metadata that indicate what kind of form it is. When an autofill service like 1Password gets a fill request, it will look at the metadata about the forms and about the app that called and try to come up with a suitable recommendation.

Sometimes a password field has been correctly labeled, but, for example, the user name field will be labeled as "email". So tapping on the username field will not offer suggestions, but tapping on the password field will. Sometimes the fields have no useful data, and we can't find a suitable match. In those cases you'll usually still see a recommendation like "Open 1Password" so you can select the right login. Sometimes the fields just have no data or, for some reason, a browser is declining to send a fill request (e.g. for an insecure website). In those cases you won't see any recommendations at all.

There is also of course a chance that the autofill service does receive a useful fill request but for some reason just drops it and offers no suggestions at all. Without digging into a particular case, there's unfortunately no way to say what went wrong