Right. I think I know the problem. It's the FTP Locking. If it is, it will be nothing to do with iWeb
This is to do with your web hosting server. What many hosting companies do is they place a lock on your FTP settings after a certain ammount of time.
It is a well known fact, to web developers, that hackers are able to retrieve your FTP Settings and Passwords and then publish their own website (often with more viruses) on your website. To stop this, web hosting companies place FTP time limits so after a some time, which is customisable, your FTP settings are not valid. So basically, to put this into English, it makes your FTP settings invalid, and is only valid when you unlock it.
Do you see below how I have to unlock my FTP:
So basically, you will need to unlock it from your web host server and publish while it is unlocked. Give yourself plenty of time to upload because you don't want to find out that it locked half way through your upload.
It is recommended that you keep it locked, basically because others can't publish their own websites on your servers.
Hope this helped.
Please reply if there are still a problem. I will happily answer any other questions.