That can happen, depending on your folder structure. By default, UE packages only those assets that are referenced in the levels or somewhere down the reference chain that starts from any of the levels.
There is a section in the project settings where you can specify content directories that should always be packaged, regardless of whether they’re referenced by something or not. Add your UI directory to that list and try again.
EDIT: Finished my reply before I saw your edit. Still worth a try.