SFTP Doesn't Work

I setup a new site on our Managed Wordpress account and the site is working fine, but I am unable to use FTP. I've created an SFTP account in my local FTP client, Transmit. At first it worked. Then I logged out and tried to login again, but it said there was an issue with the username and password. I was locked out for hours. Then I tried the next day, and it just worked (I didn't change anything). After uploading a few files, I did the same thing - I logged out and tried to log back in, but again, it said there was an issue with the user name or password. I then deleted the entry in Transmit, added it back again, and it worked. A third time, I uploaded some file, logged out and tried to log back in, and got the same error. It's like it only works 10% of the time. What is going on?