Dropdown Menus problem

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I'm working on a clients' site and having a heck of a problem with the dropdown menus, i.e. Quick Links, Search, etc. They work fine in Firefox, on both the default and custom theme that I'm designing. But in IE7, they work on the default skin, but not on the style I'm designing.

This is baffling to me, because I have not edited the navbar template at all. In trying to troubleshoot this issue, I copied the navbar template contents from the default vB skin (since it works in IE7) and pasted it to the navbar template of my custom style, to see if it would work in IE7. NO, it doesn't work.

Does anyone have any ideas on this at all?? I'm at a loss.