Gamerscore not updating on Xbox one

The issue is that when you download your profile on 360, for some reason it forgets you have anything on Xbox One so it does not display that, hence it looks like you have a lower gamerscore on 360. The way to force it to update is to earn an achievement
on Xbox One, then go back to your 360. Your 360 should just update to include all your gamerscore from One.

Update - cleared the cache 3 times, turned Xbox 360 on and off, gamerscore still doesn't acknowledge my Xbox One score.

Xbox.com has not been updated to show xbox one activity. Only your gamerscore will see an iincrease here on the site

Have you, by any chance, earned about 8,000G on Xbox One games?

If so, that would be the reason as, after redownloading the profile to a 360, the 360 does not always reflect the Xbox One gamerscore. If you earn another achievement on One, that should be enough to force the 360 to update its information. Fortunately,
your gamerscore is appearing correctly everywhere else, so it doesn't seem like there's anything to worry about, especially if it is Xbox One gamerscore being the total discrepancy.

Hey There!

Gamerscore will update automatically, there's nothing you need to do. Sometimes it might take a little bit, but you will get the credit.

As for the achievements posting, playing Xbox 360 games on your Xbox One will not cause achievements to post to your Xbox One feed..

For example, in a Xbox One game, you get a little achievement note in your player feed showing your unlocked achievement. You will not get that achievement feed item in your Xbox One feed for getting an Xbox 360 achievement. Your gamerscore will increase
either way, because your gamerscore is not attached to 360 or One, it is attached to your Xbox Live profile. As long as your console(s) is/are connected to the internet, your global gamerscore will increase with any achievement- One or 360.

Cheers,

Nigel