Apologies, but this forum only supports the FrostWire version 4. For the torrent only version, please visit FrostWire Community Forums • Index page
Although your comment, only the torrents vanish suggests maybe it's not FW 6 or later. We are not knowledgeable about FrostWire's torrent behavours.
It might be a question of how you are opening torrent files. Directly via the site using magnet links or via .tor files? If you are opening via magnet link, then the magnet link might be failing. I noticed this as a problem with one torrent site in particular with about a 15 to 25% success rate. And your problem has all the hallmarks of a failed magnet link download!
If possible, try to first download the torrent file (.tor) and open that file with FW. (If that torrent site does not offer .tor downloads, see if you can find a mirror site also hosting the same torrent. Some torrent sites will have a page or section showing mirror sites for a particular torrent.)

I just tried opening a torrent file (.tor) downloaded from a torrent site and no problem starting the torrent download, although it took a few minutes. Another appears to have no seeds (these torrent (.tor) files were over 2 years old.)

If you are using FrostWire 4, then hopefully you do not see a brick wall icon near the connection icon bottom left of the FW screen.