Shopify showing « payment is pending on PayPal Express Checkout »

Description

Hi, this was my first order (I got 10 payments after the first one and all went through). The customer paid through Paypal and it shows ‘A xxx USD payment is pending on PayPal Express Checkout », however, the payment is not showing in Paypal. This could be an ‘echeque’ or something alike. The problem is that it has been 4 days and still no news.
The problem could be that the payment gateway was not set up properly or it was somehow stuck/pending, as sometimes happens when you set up the payment gateway in shopify.
Does anyone have a clue or indication on how to solve this? thanks.