We tested our SFTP server to be compatible with most SFTP client programs. Could you please email us the detailed information about your SFTP client? If we can duplicate the problem, then usually we can located the problem and solve it quickly. Also, please test another SFTP site: ftp.drivehq.com. It is slightly different to support different key algorithms. Please let us know if ftp.drivehq.com works for you.
If your SFTP client also supports FTPS (FTP over TLS), then you can use FTPS instead of SFTP. FTPS is as secure as SFTP.