Last payment not shown in the orders list

Hi there,
the last payment I received is not shown in the backend orders list. Payment has been correctly received and processed by Paypal but if I go to the plugin transactions list I cant’ see it. What’s wrong? I tried to use Paypal sandbox to make a test and everything went fine. What is the possible cause of this issue? How can I fix it? Is there a way to manually add the transaction to the list? I noticed there is a “new order” button on the top of the page, what is intended for? Can it be used to add the transaction manually? what is the exact procedure to follow? Thank you for your help!