mod-sftp can't replace the exist file on server

[Sense Zeng]

proftpd version: 1.3.4a
mod_sftp version: 0.9.8

I use sftp(openssh 5.9p1-5ubuntu1.1) in ubuntu connect server. When I try
to replace a exist file on server like text.txt, it printed like:

Uploading test.txt to /test.txt
remote open("/test.txt"): Permission denied

But when I remove the test.txt on the server first, and upload the new
text.txt  with nothing wrong.

Sorry for my poor English.