Sharing/uploading files to others keeps failing

A couple of users have told me that they are trying to download files from me that keep failing. I kept the client open. When I installed it I said yes to making an exception for it in regards to the firewall. My downloads are working fine.

I couldn't spot anything relevant in my FAQ or forum search. I don't know what to try next, but if it happened with two people within a day I'm guessing the problem does lie with me. I want to share the files. I just can't figure out what the problem is.

If anyone has ideas, or can even suggest good (better) search terms to find a thread on this kind of problem in the forum, I would be grateful.