when I am on the "all members" back end list and try

Hi,
when I am on the "all members" back end list and try to look for a member using the search form on top, it always gives me a no result.
It used to work find whether I put just one letter or partial name but now, it will only work if I put the exact and full name as it appears on the username column. And only that column. But most of the time, we only have partial name and it's the first or last name, not the username.

Do you know why this is happening now and it didn't before?