Filter module won't work

Hi there ! First of all thanks for providing such a great platform as Opencart!

I will try to describe my problem: I have installed opencart , customized it , and using for a while now , but lately I have discovered a weird bug , my filter module won't work the way as it meant to work, so if I choose one option it refines correctly , but if I choose two or more , it displays a messy list of items not matching chosen options.

Well you can check it yourself here http://sonet.am.

Thanks