Hi Simone!
Thanks for getting in touch with us! Let me help you with that issue.
Having a closer inspection to your site I can see there are some styles coming from your theme that are conflicting with our plugins.
In the following screenshot you can see the style that makes the dropdown menu to not display:

To confirm this, you can switch to a default theme, like twenty seventeen, and see if it shows correctly.
I’d suggest you get in touch with the theme authors to see if they can help you with this problem.
Let me know how that goes.
Best,
Victor