Sent Files are "Pending"

Hi, I have a user who has sent a handfull of files to an external contact. When he looks at his "Sent files" overview, 2 files have status "Sent Email", 1 is "Downloaded" and the last 2 are "Pending". These last 2 files have been "Pending" for 4 days now and the recipient has not received the link to the "Pending" files yet.

What does "Pending" mean in this case and how can we get them progressed?