Java APIs

I'm writing a Java distributed application that should work both as a client and as a server. I mean, suppose a lot of these apps running and connecting among themselves thru RMI; sometimes they act as clients, sometimes as servers. At first I wrote two different classes to be run on the same host, one representing the client side and the other one representing the server side of the same app. It was working fine. Now I'm trying to put the two ends altogether, and I get a StubNotFoundException trying to load the server's stub at runtime. Can anybody tell me what's wrong with this?

Thanks in advance.