Maybe no more Promo tickets are available for that train? Have you tried for different train schedules?
I just took a look at a date 1 month ahead and promo tickets were available for 4 people in all but one train that day.
There is a disclaimer at the bottom of the page where trains are listed to the effect that tickets are subject to availability and that the offer may even be sold before you complete your purchase eg if someone else gets there first. If price is more important than timing, check trains before and after your preferred time.
On the way back I only received 1 full discount ticket. Train departs Porto at 6:30pm. When I booked my seat the website shows only 4-5 seats are akkocated.
Therefore the preliminary observation is CP discount like airline tickets very much depending on the peak or off peak time.