There are two possiblities
1.if you testing payment from localhost it not work
2.You must have to enable auto return in your PayPal account, otherwise it will ignore the return field.
you can enable using those steps ..
go to your PayPal merchant account-->profile-->Selling Preferences-->Website Payment Preferences-->Auto Return for Website Payments turn radio button to on,default it should off.
Documentation : https://www.x.com/sites/default/files/ipnguide.pdf