ftp filter exception problem

Hi all: I am having an issue with my ISP when connecting
via FTP. We run BM 3.9 sp2 and are using the
ftp-port-pasv-st filter. I can connect to the ftp server
OK, but cannot get directory listings. My ftp client says:

Failed to retrieve directory listing

Turning of filtering (unload ipflt) resolves the problem so
it has got to be a filter issue. I suppose I can create
inbound and outbound filters for ports 20 and 21, but
shouldn't the stock filter work???

Thanks for the help, Chris.