One of my affiliates shared the referral link, but no commissions were generated when the customer purchases, how can it be?

When such an issue occurs, it depends on the cookies, and specifically, the first visit is stored in a cookie that for some reason changes when the user finalizes the purchase.

Let’s see some examples of how the cookie info can change:

• The user visits the site through one browser for the first time, where the cookie is stored, and then leaves. Then, he/she visits the site through another browser and completes the purchase. 
• The user visits the referral link of affiliate A first and leaves, but after a couple of days, he/she visits the site through the referral link of affiliate B and finalizes the purchase. The commission is credited to affiliate B instead of A.
• The user is associated to the affiliate but the Prevent referral switch option is disabled. As in the previous case, the commission will go to affiliate B instead of A.