You need to also allow port 20.
FTP uses both port 20 (For data transfer) and port 21 (for control).
I think you may also need to allow connections to a range of high port numbers, depending on if you are using passive or active FTP.
I found that a "better" solution was to set up SSH (port 22). Having set this up, you can use sftp, which, apart from being more secure, is easier to set up.
I hope this helps