There are several reasons why form filling might not work as you expected.
1. You are trying to fill (save) a Flash form or Java form or other non-HTML form. Web Replay only fills HTML forms and NT Authentication forms. You can check which the form is by right clicking it.
2. If you are filling form Web Replay toolbar and some fields are not filled, this might be caused by changes to the web page. You have to delete the login and later re-create it when the detect login dialog will pop up.