It's speculation on my part, but from what I've observed uploads that have been queued but that are now eligible to start transferring data are removed if they don't start after a minute or so. That is to say, the empty upload slot is held for about a minute. If the client at the head of the queue doesn't start using it during that time period, they lose their chance.

I wonder if Shareaza has problems with time-management at its end, causing it to sometimes miss it's window of opportunity to start downloading data ?