Sounds like more arp destroying that poisoning. Arp poisoning works by kind of cunfusing the network and routing information through a poisoning device before it is properly sent on its way to the correct machines. What are you trying to accomplish by "trying to communicate with the ftp server"? Or you trying to intercept packets going between machines and the ftp server? Or just doing the posioning and trying to connect to the ftp server. If you are trying to connect to stuff from the posioned computer and you are failing, then the posioning is busted, or hardware is crapping out, else it would route packets correctly, just as if there was no poisoning going on.