Well, it was thread about something else, but anyway.
That's actually how I end up hanged on Ethereal.
Let say there are ed2k 80 servers. And let say I have download with only 3 sources (some freaky ebook I'm fourth person in the world interested, for example).
I have searched globally for file and found it. I have started downloading it. But it doesn't start up. Why? Because aMule doesn't pick-up sources from search and request sources only from current server. What chances are that this three sources are known to my server? - 3/80 - very very low. So I added download but it is never getting any sources, since all found sources are connected to some other server.
I'm not ed2k protocol guru, as was stated above. I might be mistaken. You souldn't listen for my explanations - but only for facts they accompany

. I have, for example, presumption that "Get Sources" gets sources only from current server.
For some reasons (I was unable to identify) aMule is able to get sources on relaunch. Ethereal log with server I'm connecting to has no search nor get sources request.
Okay, I relaunched aMule and it had found (God knows where from) this 3 sources. As source goes offline/ip address changes - it is dropped. And since it is reconnects not to my server, I'm not able to see it ever again. So I have started with three sources, and e.g. in 24 hours I got zero. So I have to relaunch aMule again so it will search again for this file again and pick those 3 sources again.
And with rare file, there is no other downloaders - so all benefits of source exchange generally applicapable to popular files are negated here: I'm the only downloader and I'm the only keeper of sources. I doubt uploader cares about downloaders.
Solution could be that: i) if file is downloaded from just performed search - aMule sould pick sources from the search; ii) if for some time we are getting no sources for file from connected server (and source exchage is not available - no other downloaders), aMule should perform search/global search.
Something like that. Second part sounds huge, but first I hope in most situations would be suffiecient. Most of rare files are small. Once I tried to download iso image from singe source who was about 2 hours per day on-line - that was horrible, I do not do such crazy things anymore

P.S. The option "keep 5 sources for rare downloads" was activated all the time - I have no clue why it didn't worked for me. And to be honest, have no clue what it does.
P.P.S. I can assure you that I had waited long enough. Some downloads were sitting with no sources for more than day, since I didn't wanted to interrupt other ongoing downlods.