ftp write access on proxy sever nt 4

Hi,i am woreking on an nt 4 (small business server)running proxy server,and although the server itself can gain ftp write access to websites,none of the clients can?,it gives an error such as “the proxy server will not alow access”or such like,i have added users to the allow them ftp access opened port 20 and 21 but still no joy,i am wondering if it is possible to allow this to happen on proxy server as all i can see is ftp read acces on one of the menus?any help in this regard would be very much appreciated
regards

Gary