Hi,
Make sure to clear your cache afterwards, also, if you have any other search plugins active that may affect that page, I suggest disabling those as well – as multiple search overrides may not work at the same time.
If that does not work, there might be a custom search query hard coded to the theme, which cannot be overridden. In that case, I can take a look at the theme search template file via FTP, I may be able to disable that.