SuSE and VSFTP

I'm having some fun with SuSE and FTP. If the firewall on the server is turned off, the client works perfectly happily. But if the firewall on the server is turned on, it won't. I have opened up both 20 and 21 TCP and UDP to a limited IP range (within my network), but it just won't have any of it. I can log in, but I can't ls or change directory. Anyone got any suggestions? Many thanks in advance. D.