I'm currently investigating a problem on a WordPress website where sub-menus are not showing up after upgrading to WP 3.9.1. The website, which can be found here, is using the Zeus theme (v. 1.1.0) and it seems that the behavior of the sub-menus is controlled by "jqueryslidemenu.js".
After the WordPress upgrade, the version of jQuery loaded changed to 1.11.0 and it looks like this theme was created with an older jQuery version. This leads me to believe that this might be the cause of the issue. Interestingly, by inspecting with Firebug, I noticed that the sub-menus are being generated within <ul class="submenu">
- for example when hovering over "About" in the main navigation menu - but they remain invisible. So I am hopeful that there is a simple solution through editing some JavaScript/CSS.