The fact that you have a seedbox doesn't mean that you will upload with full speed all the time. First you must understand how the bittorent protocol works and realise that seeding from a seedbox is the same as seeding from your home PC. The only difference is that a seedbox works 24h and is connected to a very fast network. Therefore if a torrent doesn't have any peers or there are a lot of seeds and only a few peers, where the seeds:peers ratio is e.g. 8:1, you won't be able to upload fast or upload anything at all. This is how the bittorrent protocol works and a seedbox won't work wonders to get around these rules.

Speed drops when a torrent is already downloaded
The best speed you will get when you are downloading a torrent, bittorrent clients prefer to upload data to other bittorrent clients from which they downloading. Another important thing is that on most trackers people use seedboxes and when you/they finish downloading, mostly home users remain. They are not able to download as soon as seedbox users and there are a few seeds with a fast internet connection. Then the speed with which all peers can download is divided into all seeds and you must remember that peers exchange data while downloading (described above) and that's why after downloading is finished, upload speed drops.

 
I have downloaded a few torrents and upload speed is near 0
The worst thing which you can do when you have bought a seedbox is go to a tracker and downlaod old torrents. This is a mistake made most frequently by people who have just begun their adventure with bittorrent or seedboxes.
You must understand how the bittorent protocol works and realise that seeding from a seedbox is the same as seeding from your home PC.The only difference is that a seedbox works 24h and is connected to a very fast network.

 
Tracker reports a lot of peers but I can't connect to them
It happens very often that people don't download all the files from a given torrent but just a part of it, and next they seed to improve the ratio or they just don't need the whole of it. Then the tracker shows e.g. there are 200 peers but, in fact, there may be no-one there because they have downloaded a part of the file so now they are seeding and nobody is downloading this torrent.