The value you have for the auto return should be the page the user is sent back to when they transact. I did find this note though in the details for this setting:
Note: If you have turned on Auto Return and have chosen to turn on PayPal Account Optional for new users, a new user will not be automatically directed back to your website, but will be given the option to return.
So this may be interfering with the return for some users. As a test you should try not setting the return URL in the buttons themselves and just see if PayPal will make the return for your. If the return is not happening for you at this point you should contact PayPal and have them review the process.
