Question : Problem: ssh works, ftp works, but sftp doesn't work

i have a standard debian-based lamp server running. ssh and ftp both work fine, but when i try to connect via sftp it doesn't connect. can anyone help?

OpenSSH_4.3p2 Debian-9etch3, SSH protocols 1.5/2.0, OpenSSL 0x0090803f
 - ProFTPD Version 1.3.1

Answer : Problem: ssh works, ftp works, but sftp doesn't work

... and /usr/libexec/sftp-server exists and is executable?