WP Network, Upload folder is not writeable….

Hello,
I have a network installation of WP, the main .com has no problems, but all of the sub-sites have the following error showing up – “Upload folder is not writable. Export and file upload features will not be functional.”

For example:
website.com = no error
website.com/sub-site = error

I have tried changing permissions for all of the associated folders, but I just can’t get this to go away.

What am I missing and how can I fix it? Thanks for any help!

Tom