@idumy This gets really complicated. I "nearly" had an answer and was going to post an "info guide", then I discovered a whole other layer which has made me pause and rethink.
In simple terms, at least one person in a swarm has to be contactable to start the torrent "conversation". This translates to having their port-forwarding enabled and correctly set up in the client. Most of us hide behind a VPN. Most VPNs don't support port forwarding.
The amount of seeders and leechers in a swarm is, in theory, irrelevant, except that the more there are, the more likely one has the port forwarding capability needed for everything to start working. The only solutions I've found so far is to switch to a VPN provider who does have a port forwarding feature, or to risk running a torrent client without that protection, exposing your real IP to the world.
I've recently tried this with another member. I'd been theoretically seeding several torrents for ages yet, even though there had also been one leecher connected for ages, nothing ever transferred. I installed a totally separate client to ensure clear separation and turned off my main client, then turned off my VPN and added four of the problem torrents to it - three I was seeding and one I was leeching but had been stuck on for weeks. The three I was seeding immediately began transferring and ran through to completion, and the other member confirmed he'd successfully grabbed them. Unfortunately this didn't solve my own leeching requirement. Having completed this test, I reverted everything back to normal.
A day or two later, suddenly around twenty similarly stuck torrents came to life, including the one I was trying to leech. On checking, I learned that the other member had tried temporarily switching to another VPN - Proton - who DO support port forwarding.
This explanation is also supported by something else I couldn't previously explain. When I upload new torrents, they can often take two, three, or even more hours before they actually start seeding. These are fairly niche torrents - Asian BL - so only a small amount of takers. The time taken is more to do with the number of leechers. There often needs to be four or five before it starts. Once it does, it's full speed ahead. Sometimes, for no apparent explanation until now, the new torrent starts pretty much as soon as there's one leecher. I can only conclude that one leecher does have port forwarding enabled and working.
Sorry that the above is so detailed! This issue has been bugging me for an awful long time, and I don't think it's something many are aware of, so I thought it information worth sharing. Unfortunately, I've now learned that it's not "the" answer I've been seeking, as there are a couple of other factors to consider. I've currently paused in my search for understanding, as I was a little worried my head would explode!
Please understand that I'm NOT suggesting just turning your VPN off for everything. We use them for a reason, and torrenting without this privacy shield is extremely risky. But, if there's something you're desperate for and you're notionally exposing yourself ( ) to only one or two seeders, you may consider it a risk worth taking in that instance. I do, though, recommend doing this one or two torrents at a time, and using a different client dedicated to solely that purpose, to minimise the risk of accidentally torrenting as usual while you're naked for all the world to see!
