Responsive layout, menu doesn't work with hikashop

I use a web-responsive layout.
Everything work fine till I install hikashop. Just install it nothing else. When Hikashop is installed, both on mobile and tablet, the responsive design adjust the main menu at top in the responsive way with the icon menu, but when i click on it, no drop down list of menu voices is diplayed.
If I disinstall Hikashop it works fine again.

Don't see javascript errors in the firebug console. What could it be? Some CSS conflicts with Hikashop?

Thanks in advice