SSH key pairs not working with Serv-U

I cannot get  any DSA 2048 SSH keys that are password protected to work with Serv-U FTP server, FTP Voyager (client) and CuteFTP (client).

Here's what I've tried so far:

• Created non-password protected keys with FTP Voyager. They work with both clients.
• Created password protected keys with FTP Voyager. They don't work with either client.
• Created password protected keys with CuteFTP. They don't work with either client.
• Created non-password protected keys with CuteFTP. They don't work with either client.
• Created password protected keys with Putty. They work with FTP Voyager. CuteFTP keeps asking for the password even though the correct password is already specified in the CuteFTP settings. I just keep getting the same prompt each time I enter the correct password.

What else can I try and what's causing this issue?