Menu bar does not appear on the smart phone version of the theme.

Hi guys, i have the “december” directory child theme installed, but when my site is viewed from a mobile device like a smart phone, the menu bar does not appear,making it impossible for user to register to my site from there phones, however, the menu bar appears on the PC version of my site. This is a big problem, because if my customer can only register with a PC, thats going to cost my a large amount of users who only access the internet with there mobile devices. Please any ideas?
Thanks.