You'll need to go through the options for seeding - you probably have it set to stop seeding when it reaches a certain ratio or had it running for a certain amount of time. Just change it to something less restrictive - I think there might have been one to make it seed indefinitely (except when you manually stop it, of course), but I don't have Azureus in front of me to check right now

Alternatively, try right-clicking the torrent and use the Force Start instead of normal start option if you only want that function occasionally and not for all torrents