In qbit click on the stalled torrent and look for "last time seen complete" if it says "never" it means the file is dead. None of the seeders have the last bit of the file and the seeders are other people waiting for somebody to complete the file.
If this happens just delete and restart from a different source.
Edit: This applies to files that have started downloading then stalled out later. If your download is still at 0% you have a separate issue.
Not all seeds are online 24/7. Sometimes leaving the torrent running for hours or days can allow you to download it when that PC/server gets switched on.
Okay but there are plenty of real-world reasons that you might not be able to download from those seeds.
No spare bandwidth. Their bandwidth might be saturated on other peers, other torrents, or other applications entirely.
Bandwidth limits. All torrent clients have ways to limit bandwidth. Most often have a UI button to switch between a high and low option. Some also have schedulers for that as well.
Connection limits. All torrent clients have a maximum limit for open connections (ie peers) because large amounts can be very taxing on resources. There are also generally ‘upload slot’ limits too, further limiting those connections that can be used for seeding.
ISP Port throttling. ISPs are constantly trying to block P2P traffic, the seed might need to change their port to become unblocked.
Misconfigured UPnP/Port Forwarding/Firewall settings blocking the seed’s incoming connections, while it still reports to the tracker as this is an outgoing connection.
Geographical distance can also make a huge difference.
Public trackers amplify some of these issues more than private ones. Hence why you often see network configuration knowledge needed for interviews.