I wrestled with this issue for some time before figuring out the cause, so I hope this helps someone out there. I had a JComboBox with a custom ComboBoxModel. Everything was working fine, except a call to setSelectedItem would do everything it should (fire events, update the selected item property) except it wasn't updating the displayed value in the JComboBox itself. Clicking the drop-down would even show the correct item selected, and getSelectedItem() returned the correct result; it was just the box itself that was wrong.
Apparently the displayed value in the JComboBox isn't based on getSelectedItem(), but rather on the top item in the list. I don't know how or why this is, or if it's due to some intricacies of my GUI code, but bringing the selected item to the top of the ComboBoxModel's item list when calling setSelectedItem fixed the issue. Go figure.
If anyone has any insight into what causes this, please drop a comment!