mobile menu problem

I am trying to use simply static with my basic web site at this location and all seems to work well except for if I try to access the site from a mobile device in portrait mode. If I try the site on my phone, the two line link to a dropdown menu does not respond. If I rotate the phone to landscape then the menu choices come back and things work. I suspect I have made a simple mistake, but am not sure what it is.

The same thing happens on a regular browser if I shrink the page to “mobile size”

The page I need help with: [log in to see the link]