menu showing but not opening on site with custom theme

hi, thanks for this lovely plugin! i have seen it on different sites, and know it works realy beautifully. now i just wanted to install it on a site with a custom theme and it shows, but doesnt work. if i switch to twenty eleven, it works. i just cant find the error. can you help? thanks! nadia

https://www.ads-software.com/plugins/responsive-menu/