Paypal API integration issue

What I am doing ? I have 2 Paypal Accounts. First belongs to my client and second belongs to me .On the first account I have created the Application and got the Sandbox credentials(ClientID and Secret). Then I configured Paypal API in my Laravel 5.1 application and set the Sandbox credentials. So far everything is fine and I am able to reach the paypal page for the payment using the Sandbox credentials. What's the problem ? When I try to do the payment, it always says wrong credentials. Although I am able to login into my Paypal account but when I am on the Paypal payment page and tried to do authentication, I always get authentication failed error. Question Am I missing something ?