Getting "520 FTP: command not valid during data connect

Hi all. Everytime I try to upload a few files into Smart FTP, one will upload, and then nothing happens until I get the message "520 FTP: command not valid during data connection". Every time this happens, and I have to manually upload files in small groups, until it happens again, and then I close and reopen the connection. In the end, I find out files haven't uploaded and it gets really frustrating. Why is this occuring?

Thanks

Josh