When a page will not fill
Some tabs cannot be filled. Others can, but no field matches the profile. The status line in the popup tells those cases apart.
Chrome blocks the tab
Extensions cannot run on Chrome’s internal pages, on other extension pages, or on the Chrome Web Store. Open a normal website and try again. The status says when the page is restricted.
No matching fields
A page of plain text has nothing to fill. A form that uses unusual labels may need a custom field or a rule. Empty values in the profile are skipped, so a blank email in the profile leaves the email field alone.
A field you already filled
If skip-filled is on, existing text stays. Turn it off only when you mean to replace those values.
The page still clears the value
Reload the page and fill again before you type. If one site always wipes the field, fill that box by hand and leave a rule off for it. The rest of the form can still come from the profile.