[Plugin: WPSearch] Not all results showing

If I search for ‘cosmo’ it should display ‘cosmopolitan’ and ‘cosmogirl’ but it doesn’t. http://www.abonnement.nl/search/cosmo

But if I search for the full title then it will give me the results. http://www.abonnement.nl/search/cosmopolitan
http://www.abonnement.nl/search/cosmogirl

these are the posts:
http://www.abonnement.nl/tijdschriften/cosmopolitan/
http://www.abonnement.nl/tijdschriften/cosmogirl/

WPSearch 2 by oconf.org