Thanks @Nilim and @TKF but I verified and re-installed several time to try to solve this issue, but it didn't change anything.
From what I understand, the POS purchases are classified as "ORDERS".
On a contact page, by default, it shows the numbers of "SALES" and "AMOUNT INVOICED", but it does not show "ORDERS".
So even when I select a contact in the POS, process his/her purchase, it doesn't show on that contact page.
Also, when I choose the option "INVOICE" in POS, it create an invoice, but doesn't record POS payment for it, so i can't give an invoice to a client that just payed at the tilt.
(In my case, computer goods and services. I need to print not only a receipt, but also an invoice for warranty/delivery purposes)