In general, if the package showing 'successfully delivered', it means the recipient has got it. If you didn’t get it, better first check P.O box or ask your friends or neighbors. One of them may receive it for you.
Another situation is the courier delivered it to the wrong address. You need to contact the seller to confirm if the tracking number is right. The seller may give you a tracking number of another customer. If the number is right, he may filled the address by mistake. If all are right, there is a chance that your local courier sent it to a wrong address by mistake or it was stolen after the courier delivered to your address. You need to contact the courier who delivered it in your country to reflect the problem and check it for you as soon as possible. If the package couldn't be found, you have the right to ask for refund or require to send one again to you from the seller.