Order is not being placed via prestashop paypal shopping module

[umeshinator]

Hello,

While shopping and paying through prestashop paypal shopping module,after paying through sandbox paypal account successfully,order needs to be in the server but while going back message appears

"

ORDER HISTORY

Here are the orders you have placed since your account was created.

If you have just placed an order, it may take a few minutes for it to be validated. If your order does not appear please refresh the page.

You have not placed any orders. "

 

which simply means order has not been places and after debugging i found that notify_url was not called by the paypal,which has made order process and order detail unavailable in database and similarly in prestashop backend, I could not sort out the problem,can anybody help me with this? I would be very grateful ,thanks in advance