There's a bug in the combo box plugin where, after clicking on the combo box, the list will immediately go back up. This only happens when there are items inside. If you have an empty combo box, the list will still open. The CAP file demonstrates this.
The second one opens for me; in any case we only implement a standard combobox, so I can't imagine what the problem could be.
What OS do you use?
Windows 7, but Vista has the problem as well. Haven't tested it in XP.