For some reason, Shareaza seems to treat a "busy" from a push source as if it were a 404 error: it promptly forgets the source and won't retry getting the file automatically. If the source is manually re-added, eventually the file downloads.
Shareaza should automatically continue trying to get a file when the cause of its failure is temporary, such as the remote source being busy. It especially should do so if Downloads.NeverDrop is turned on, but that setting has no effect on the forgetting-busy-push-sources behavior.
P.S. Would whoever runs this forum mind making the little "remember me" tickbox on the login form actually perform as advertised? Having to log in over and over again every few hours is getting rather tedious. Every other site I use either just stays logged in for days if I don't explicitly log out or else has a "remember me" or "stay signed in" or similar tickbox and stays logged in for days if it's ticked. This one won't stay logged in longer than a few hours (and the amount of time seems to be somewhat random!) no matter what is ticked. As far as I am concerned, this constitutes a deviation from a de facto Web 2.0 industry standard practice, and it's a deviation in a user-annoying direction, so, a bad deviation. And a deviation apparently lacking in justification. (I assume it's a software/configuration issue rather than deliberate, since it doesn't make sense for it to be deliberate.)