I am having trouble with the mobile menu in my 3 layer navigation as the sub menu keeps disappearing whenever I attempt to hover over or select a link.

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It could be due to a few reasons:

1. CSS: Check your CSS to ensure that the sub-menu is set to display: block; when the parent element is hovered over or selected.

2. JavaScript: If you're using JavaScript to toggle the menu, ensure that the event handlers are working properly and that the sub-menu is not being hidden by mistake.

3. Z-index: Make sure that the sub-menu has a higher z-index value than the rest of the content on the page so that it appears above everything else.

4. Responsive design: If you're using a responsive design, make sure that the hover event is being replaced by a click event for mobile devices.

5. Server issues: It may be due to server issues, causing the menu to load improperly. Try clearing your browser cache or using a different browser to see if the issue persists.