One other thing that you might try....
back up and save only those files that are neccessary for each user. Then delete the user accounts WITHOUT saving the files contained...you just backed up the important files and can reinstall them.

Recreate the user accounts without the backup files. See if the flash player then works for each user account as it will be embedded in the working browser already...it should work with each user.

If no go...then try to download the flash player to each user's browser.

if the flash works then reload the backup files and see if there is something they did to stop the program from working!

Keep us posted!