Some FTP commands failing - SOLVED

[#1]

I can log in to the FTP server, and some commands (pwd, cd) work, but others hang, time out, and fail (dir, put). 'Put' does remove the existing file at the far end, but replaces it with an empty file, not the one I want to transfer. For more details, see the attached screencap.
Is this a known problem? Is someone working on it?
Edit: subject changed to show problem is solved.

[#2]

It looks as if this is my problem. If I understand correctly, Port 21 is used for control, Port 20 for data, so it looks as if something at my end is blocking Port 20....

[#3]

Solved! PN's firewall blocks Port 20 on its 'High' setting. Simple! 

[#4]

Did you try passive mode? I think that should work through the firewall.

[#5]

Yes, I tried the PASV command, which the server accepted, but for some reason it made no difference. Turning the firewall off briefly sorted the problem.