First, make sure you can see your list of overrides, see this article if your language overrides list is empty, but you are sure there should be some there.
If you have used the search box, try searching for only a short part of the text (string). Sometimes if you search for the whole string it won't find it because there are unexpected characters in there. In this one we have the word 'registered' in italics - between the i tags <i> & </i> so searching for
"Please login if you have shopped and registered with us before, otherwise please complete the form below to register for the first time."
would not have been found.
Then make sure you have gone through all of the pages using the pagination links at the bottom
OR listed all of the overrides on one page.