Hello Dan,
Sorry to hear you are having issues with your Primas theme on mobile version. We tested the theme on phone and do see an issue. The issue is that the submenu is appearing, but it is white on white, so white text on a white background. That is not a good thing.
In working with the dev team, we did find a temporary fix while they work on the issue for the next theme update.
Using the Advanced Tab in the Customizer, select the Advanced JS&CSS button.
Next, select the button to add Custom CSS.
Enter the following code:
@media (max-width: 767px) {.palette-primary .navbar-default .navbar-nav .dropdown .dropdown-menu > li > a { color: #000; }}
This will make the text of the submenu black for screens smaller than 767 pixels wide. That should work until the dev team gets the updated menu released.
Kindest Regards,
Scott M