Xbox Live Prepaid 24 Month not working

It will be converted to Ultimate, but not at the full rate. After you've taken the $1 (or 1 local currency unit) offer that converts your prepaid memberships at parity (that is, one month for every prepaid month), subsequent prepaid cards will be redeemed
at a lower rate. You can find a conversion chart in the
FAQ on the Game Pass Ultimate page. But for quick reference, 24 months of Xbox Live Gold will be converted to 8 months of Ultimate.

You can add the 24 months of Ultimate to your existing subscription as long as it won't go over 36 months. If it were to go over the 36 months, you would need to wait until you have 12 months or less remaining on your subscription before redeeming the 24
months of Ultimate.

https://www.xbox.com/en-US/xbox-all-access#faq

An Xbox All Access purchase includes 24 months of Xbox Game Pass Ultimate. So, what happens if I have an existing Xbox Game Pass and/or Xbox Live Gold membership when I purchase a console with Xbox All Access?

If you're already an Xbox Live Gold or Xbox Game Pass member, any days remaining in your membership(s) will be converted to Xbox Game Pass Ultimate at a conversion ratio when you redeem the 24-month Xbox Game Pass Ultimate membership and added to your Xbox
Game Pass Ultimate membership up to a maximum of 36 months. Xbox Game Pass Ultimate memberships are limited to 36 months.

If you’re already an Xbox Game Pass Ultimate member, wait until your existing membership has a prepaid balance that is less than 12 months before redeeming the 24-month Ultimate membership through Xbox All Access. You will be unable to redeem with more than
12 prepaid months of Xbox Game Pass Ultimate.

In my Services & subscriptions page I can see:

Subscription

Prepaid 3M Xbox LIVE Gold

Expires on

3/24/2016

Paid with

Prepaid card

Hi there,

-

they would add up, but keep in mind that you cant redeem more then 24 months in the future (as far as I know it's 24 months)

So adding to 3 month prepaid cards would result in 6 months, but you can not add to 12+2 (14) month cards because that would be more then 24 months.

Hi there,

as far as I know the maximum that can be stacked as prepaid is 24 months (but I've heard others say 36 months).