AppleTV free trial not working

Only one offer per Family, regardless of number of devices purchased. Four devices or one device, it is all the same for this promotion. Use any one eligible device in the family for that, but the first redeemed one counts.

The offer must be claimed in the Apple TV app within 3 months after first setting up the new device.

Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple

It was working for a few months, and now it prompts us to pay.

• Does this happen for all the devices, all the accounts of the group, and for any Apple TV+ content?
• See if the subscription is still listed as active on the account that redeemed. See your subscription overview - Apple. If the subscription was canceled, then the remainder of the offer was forfeited.
• See if the selected content title is part of Apple TV+. Other content in the Apple TV app would not be covered by the subscription, and may prompt for a payment not related to Apple TV+.
• See if the device used when prompted, is still signed in to that device and/or app, with an account that is part of the Family Sharing group. Especially for Macs, the TV app needs its own additional sign-in (menu Account).

• See if your Apple TV+ subscription is still listed as active: See your subscription overview - Apple. That would be for the account that first redeemed the offer, and signed up for it.

That would indicate that it was canceled (or never activated) for that account.

Only the Apple Support staff would be able to look at details of your account and your Family Sharing group to be able to say more about it. Try getting back at them. You may get a different advisor this time, one that uses less quote characters.