I have this problem occasionally, and I think it's due to the fact that I am not kind to freeloaders. In Tools>Options, you can set freeloading levels, which will block people who are taking more away from the network than they give back (not sharing any files). It might be possible that you have your levels on a high setting. Since there are many, many freeloaders on Gnutella, it is quite possible that freeloaders are attempting to download from you. In this case, LW will say "File Not Found" even when it is there.

Also, you should keep in mind that many downloads and uploads on Gnutella are unsuccessful because of its instability. This is caused mostly by a lack of a central server to locate mirrors (alternate download locations) and the presence of many finicky modem users (In this I am referring to unexpected disconnections, not the practices of individual users. But that's another story).

So, unless all of your uploads are failing when you have the freeloading features turned on, there should be nothing to worry about. Hope this helps you out. :-)