- Most likely: You or they don't have port-forwarding enabled. How to tell? No incoming connections in peers tab even after a long time. Negatively impacts speed and connections. IPv6 has no limitations, but it's almost impossible with IPv4 nowadays. To do it with IPv4 you then need a VPN with such option.
- You have modified the connection settings, allow more slots: Settings > Connections > Max. simultaneous peers
- Unlikely, but happens in the wild: Someone is poisoning the tracker/DHT and inserting fake peers.