My 360 games won’t unlock achievements on Xbox one

I think you are on to something with the 360 game being the cause of the issue. I haven't been able to unlock achievements in any Xbox One game since playing the 360 game Vanquish.

This is the only variable I believe is contributing to the cause. My neighbors don't report any achievement issues, but I suspect they don't play Xbox 360 games on their Xbox One.

Are there any volunteers in the forums who want to test this theory for us? All you need to do is unlock an XBox 360 game achievement, then try to unlock an XBox One game achievement afterwards.

Achievements should be syncing up properly again now.

well all the achievements i unlocked on 360 yesterday are unlocked in my list but not counted in the tally for the game on 360 or my gamerscore anywhere but on my 360 so my gamerscore on 360 is ahead of everywhere else. even unlocked an achievement this
morning and that one isn't counting anywhere it would seem. it shows on my xbox one but hasn't counted in my gamerscore so what the hell is going on is anyone's guess!

There is no one to contact over this. A gamerscore reset is forever, and cannot be undone.

Who can I contact to undo the reset? Because I modded when i was young I didnt think it would affect it later on.

How do you get another chance to what ? Reset your gamerscore? If you modded your score then enforcement probably adjusted it accordingly. You will have to earn them the regular way from now on