Here are a few things you may not have tried on the misbehaving computer(s), ignore those that you have tried:
1. In Administrative Tools > Services, make sure the Server service has been started and is set to Automatic.
2. Can you browse to the shared folders by typing Link Removed name\folder name into the Address bar of an Explorer window?
3. Have you tried ‘unsharing’, then sharing the files again and then restarting the computer. Have the two little people appeared on the icon of the shared folder(s)?
4. Make sure the date and time on all the computers is the same as this can prevent sharing.
5. Could you have exceeded the maximum number of users of a shared folder?
6. If you use passwords, they must be identical...