mobile menu not working – pages navigation menu?

After installing the salejunction and woocommerce on WordPress 4.2.4 I face a big problem with the mobile-menu because it is not showing up.

The salejunction theme is responsive so when the screen is smaller the “pages navigation menu” appears on top. Unfortunatly upon clicking on it nothing happens, no menu shows up! I just set up one menu which is also the main-menu on this site, but it isn′t shown upon clicking “pages navigation menu”

Do you have any hints to fix this issue?