A: There are several possibilities:

Each Gnutella user has a limited number of slots reserved for uploads. When these slots are filled, you'll see a message that LimeWire is waiting for a certain number of "busy" clients. LimeWire will automatically retry downloads from these clients after a variable period of time in hopes that upload slots will become free. However, if the person who has the file you're trying to download has too many people requesting files, he won't be able to serve any more file requests, and your connection will be refused.
If the person who has the file you're trying to download has recently turned off his computer or has disconnected from the Gnutella network, you will be unable to download the file.
If the person who has the file you're trying to download is running inferior or buggy Gnutella software, the download could fail for any number of reasons.
If the person who has the file you're trying to download has since deleted the file or moved it, you may encounter problems downloading the file.
To minimize the chances of failed downloads, make your search more specific.You will have more pertinent search results from which to choose a download.