Responsive menu not appearing on all phones

Hi,

I have a website setup at https://www.aspirerwanda.org and am having trouble with the responsive menu – on a computer when you resize the window it makes the menu change from the regular menu to the responsive menu, but when I look at the website on a phone (I have tested on iPhone 6 plus only) it still shows the regular menu, not the responsive one.

Is there a setting that needs to be changed for larger screen phones? Or is something broken in my theme?

Thanks,
Peter