Reasons the files might have 'Need More Sources' status?
(1) This is a status shown in LW 4, not LW 5 which uses different terms. The official LW site FAQ mostly only refers to LW 5 now.
(2) The person with the original file might be offline.
(3) The person with the file may have all their upload slots busy.
(4) The person with the file might not be reachable.
(5) The file might be fake or worse.

The Resume/Find More sources function is not especially great. I suspect it only attempts to find addresses of those that LW was either previously downloading from or previously found in search results. (Keeping in mind dynamic ip addresses.)

Solutions?
Best one is to manually re-search the file. Then use the Get More Results option by Control-clicking on the search tab. Also try varying your searches. When possible, try at different times of day.

I'm guessing you are using either LW 4.18 or 4.16.

I am running LW 4.18 on an iMac20 with Core 2 Duo & OpSys 10.5.8 LW 4.18 appeared to be my only option for a Mac. Wrong?
Torrents found on PirateBay downloaded quickly and easily until 'upgrading' to LW Pro 2 days ago. Bb 20Mb/s which never drops below 18. Sorted out trouble getting any connection up or down. Now Turbo.

But absolutely nothing from torrents reloaded from my Downloads or Incomplete folder left over from Basic or trying new fresh torrents.
Should I dump Pro and revert to Basic? Or would the problem transfer?
Any other suggestion while staying on Pro now that it is running nicely?

PS The Mac is my "firewall' and excellent for the web but I hate its opsys and transfer files to PCs to work on them. Could live dangerously and connect PC to web but only as last resort.