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For a start, I do not recommend LW for torrents. Unfortunately LW lacks some basics for torrents. You would be better off with a program like Vuse (Azureus), or there is another good mac one (cannot remember the name.)

Could be happening for a few reasons. For a start if your share ratio is low (LW does not tell you what it is), you might be getting filtered out by other hosts who have their torrent program set to not share to people with a share ratio below a certain %.

Another reason is because torrents is very much sharing of incomplete files, you might be waiting for someone else to download parts that you are needing. The person(s) with the whole file not be online at that time. Else their upload slots or upload bandwidth is all used up (might not even be uploading that file at the particular time either). This latter reason is most likely the reason. Patience is a virtue with torrents.