Why can't I browse some users' files?
To browse a user's files, you need to be able to form a network (TCP) connection to that user, and not every two users on Soulseek can connect to each other. The Soulseek client sets up a listening port to accept incoming TCP connections. Under certain fairly common circumstances, the listening port may not be accessible to the outside world. As Soulseek attempts each peer connection in both directions, you should be able to connect to almost any other user if your listening port is accessible.
If you're behind a router, your router might be blocking traffic to your listening port. Both the original client and SoulseekQt use a protocol named UPnP to configure your router to recognize the listening port, but some routers (most notably Apple routers) don't support UPnP. Although not very common, it's also possible that your Internet service provider has your connection set up in such a way that you can't accept incoming TCP connections.
You can also try to manually configure your router to forward Soulseek's listening port. Check this list of guides to see if one is available for your router brand. To find out which port number your Soulseek client is listening on, go to Options->General options in the original Soulseek client, or Options->Login in SoulseekQt.