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.