I can totally understand that this is annoying as I can go crazy about my achievements as well (especially Dragon Age and Mass Effect). There is unfortunately not much we can do here, re-installing the content and clearing cache is basicalle the only step you can perform. You can try to delete and re-download your Gamertag but this can cause problems with your saved progress. Achievements are saved on the Xbox servers and we have no insight or access to the achievement database in general.
Cheers
Basti
