You can't move your sliders and hit "Find New Matches" right away. Actually I'm not really sure why "Find New Matches" even exists. As far as I know, your new matches are set up during the night when Dr. eH's Magic Computer runs its database search.
I have on occasion used the Find New Matches button and get new matches. It is true that the big database matching search is run during the wee hours of the morning and most of your matches show up in your inbox very early in the morning.