This is something that should work in Flex, but it is not because of some internal bug.
I had a ComboBox binded on an Array of items. Sometimes it would update the ComboBox whenever a new item was added, and sometimes it would not. The behavior just wasn’t very consistent.
There are many posts about this, but the short solution I tried was to just use an ArrayCollection() instead of an Array() as my binded source. Also, whenever I updated the ArrayCollection, I called the “refresh()” method for the ArrayCollection and also called the “validate()” method for the combobox. Try those three and see what happens!