too many variables .. but i will mention the biggest three from least to most probable

Peering - the people on the swarm you are dealing with doesn't have fast connection to download or it expensive for the service provider to reach them
so they limit traffic to these locations. however given the widespread use of seedboxes in today torrenting world this is not much of a problem because most of them are
in the netherlands. also note that the bitorrent protocol is favor the most reliable connection, means that if other seeders are more reachable to the recepients
you are in a problem. you might also notice that your best upload times are when you are also a leecher. this is also due to the bitorrent protocol
favors leechers over seeders.

Unpopular/Freeleech Torrents - once you got to the point when you downloaded the torrent. but you still not 1:1 ratio on it, you are in a situation
where you and alot of other seeders are now "looking for" people who actually need the files and it can be a problem to seed back.
because most of freeleech downloads are from fast seedboxes just like yours.

Overselling - this is the biggest factor of all. and it's unfortunately with every shared seedbox provider.
they have the interest of having as much customers as possible and you upload at 2MBs or 200MBs isn't their first priority

hope it clears things up.