The promo subscription is not automatic. The user has to redeem the offer, and sign up for it.
Previous subscribers may not be eligible for a new free trial. Could that be it?
The offer has to be redeemed on the new qualifying Apple device, not on a different device.
The offer is for the first registered user of that new device, which may not be the same person as the one who made the purchase (in case of gifts).
The offer is for Apple TV+, not for other services in the Apple TV app that may want their own subscription.
Read here how to redeem: Apple TV+ offer - Apple
Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple
Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support
↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.