I am trying to upload files using the Site Manager section of the control panel but sometimes it won't let me. Why not?
The problem is that the site manager is not really meant to be an upload tool and only has limited functionality.
All items uploaded with the site manager must be under 64K in size.
We recommend you use an actual FTP client such as WS_FTP, CuteFTP, FileZilla, etc... to upload your files. A Google search for any of those should give you plenty of information on how to get started using any one of those.
FileZilla is free, and many other FTP programs have free versions as well.