Difficult to say. Might be due to partial file sharing where you are waiting on others to download more of the file from someone they have been connected to in recent past.
I'd keep searching for fresh hosts, and use the 'Get More Results' option by right-clicking the search tab every so often. Then one trick I often use is if you see your file in the search results, cancel your download, then re-select to download from the search results. That sometimes gets it back into action.