i don't know if you can help or if this is a microsoft thing but on sonic adventure dx i dont get the achievements for unlocking metal sonic, beating all missions and getting all A rank with metal sonic. so can sega do anything or do i have to deal with microsoft?
Try deleting and re-downloading the game, clearing the game cache, deleting and re-downloading the DLC and try again. (complete an easy stage to get a quick achievement or something, etc.)
When did you originally download Sonic Adventure DX? Back in January, the file size of the game changed from 1.6 GB to 1.5 GB. Some users reported certain achievements weren't working until they downloaded the new 1.5 GB revision.