Sounds fairly usual. First time on any network, the common and popular files get downloaded quickly. As the user explores more and more, rarer files become more interesting but harder to find.

New files also attract more attention from others, and show up in "What's New" searches, so better hosts connect to you. After a while, if better souces for those files can be found, other users go elsewhere.

No matter what p2p program you use, your results will be better if your shares are in demand and give quick and easy access. Opening the ports for that program in the router means you get a much cleaner connection to the network without having to rely on all the code that tries to work around blocks like firewalls and routers.

Take the time to set your router up properly--the reaults should be worthwhile.