When you attempt to Import additional Video Clips into PrE, do you get any error messages? That is assuming that you are Importing into PrE.
What happens if you drag a clip from Windows Explorer onto one of the video tracks in the Expert timeline?
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Assuming you're in version 12, have you selected the Pin option on the Project Assets panel so that the panel stays open?
These and other things are covered in my step by step book, if your'e interested.