Changing a password (on website) does not get into my vault

I have been trying this out more thoroughly today.. I found the following guide which I had not seen yesterday (my first day). https://support.1password.com/generate-website-password/. The first method, (which was what I was trying yesterday) seems to work for some, but not all websites. Using cut and paste seems to be the most reliable!. I am not sure what the difference is but clearly relates to the website itself.