Response from ms..
Greetings, my name is Jerry with the Xbox Customer Care Advocacy Team. I would like to thank you for taking the time to contact us in regards to being unable to earn achievements. I sincerely apologize for any inconveniences that this may have caused you. Currently we are unable to provide these achievements, you may be able to contact Rock Star games regarding the issue for further support :
I sincerely apologize that I am unable to provide these achievements. Xbox support is unable to modify Gamerscore or unlock achievements. If you are having issues unlocking achievements I would recommend using a third party resource for walkthroughs and tutorials for 100% completion of games or contact the games hint/support line if applicable. We are unable to issue missing Gamerscore or achievements that have already been unlocked. Once again I apologize for this inconvenience and although we cannot honor your request, I hope you continue to enjoy the Xbox LIVE experience.
Xbox Customer Care Advocacy Specialist
Customer Advocacy & Exceptions Management Team