Who is the primary cardholder? That is the only person to whom the benefit extends. For example, if your spouse is primary cardholder, it won't benefit your MP account.

Note also that the benefit is "last seat" availability, which is not necessarily the same as "last seat for sale" availability. UA will oversell the cabin for revenue seats, but it won't oversell for an award seat. So a seat may be available for sale but not award. And yes, this is somewhat contrary to the marketing literature.

If neither of these explain your situation, then there is probably an issue with the card not being linked properly to your account.