menu on mobile devise gone

I am so happy with this theme! There is just one thing I can't find the solution for:

I use a child theme and in this child theme I've made some changes tot the menu.
On a normal screen everything looks just fine, but on a mobile phone or tablet the menu is gone.

I've tried removing all changes from my child theme, but even then the menu it not visible.
How to solve this?
Thanks for you help.