Lightroom: Why are images dimmed during import?

Trying to import a sub-folder of images into my catalog, but the preview images are dimmed and not selectable. I think this means they have already been imported, but they are not present in my catalog, nor is the sub-folder name.
Process used: The sub-folder of images I want to Import is visible in the Source panel but not present in the Catalog panel.
I click on the Import button in the lower left and the Source panel appears. I highlight the sub-folder to import. The images in the sub-folder appear dimmed in the center panel and I cannot select them. I click on the Import button on the bottom right anyway and nothing happens.
I'm sure I'm doing something wrong, but don't know what that migh be. HELP!