It’s a bit hard to see, is there more clips above your X clips? If so, you should check what is there. In general, videos should be stacked on top of the main timeline. Sound clips go mostly below.
FCPX will show the most on top video clip (if you don’t change transparency or add other effects). You can think of a video clip gets covered with the above clip. Like a stack of papers you can see the top paper but none underneath, however, there are there. If you fan out the stack, other papers will appear underneath the edges of above papers.
In your case I assume something is above what you want to show.