Gift purchased. However, but my friend has not received anything

Last week I gave two applications to a friend via Microsoft Store. Until today, however, he has received nothing. In my friend's Microsoft account, via "Buy" and pay with gift voucher, the message follows that the key is invalid. In my Microsoft account the two gifts are listed and I can read the key. The e-mail address I entered for my friend is correct. And he uses this e-mail as his Microsoft e-mail. I had given this friend an application two years ago and that application is still listed in his account. But not the two new applications. How can I solve this problem?