Issue when using lots of filters

I’ve got a strange issue: I add 6 fields (2 for first variable, 2 for second and 2 for third). When I complete only two pairs of them – it’s ok. But if I complete 3 of them, than I have “no results”. I’ve tried to add another pair of fields and had same effect: any 2 pairs work well, but any three or more – don’t work.

All fields use custom field data type.

Can you, please, help me with this problem?

http://wordpress.org/plugins/wp-custom-fields-search/