Sub-menu Not Displaying

If you could provide a link to the problem please that would help diagnose what to do.

If you’re not using a Child Theme, and if the theme doesn’t have a designated section of the dashboard for CSS modifications then install this Custom CSS Manager plugin http://ww.wp.xz.cn/plugins/custom-css-manager-plugin

Then use its “CSS Code” section of the dashboard to hold your CSS modifications:

Alternatively use your Child Theme style.css file to hold your CSS modifications:

#main-nav .menu li ul {
    top: 47px
}

Hopefully you can talk to your theme’s vendors for support for their products.