I just beat the game on solo legendary and after the credits there was no achievement. Worse, when I went to the main menu all the levels are locked except for the first which says I never played it, and the last two which show I beat the levels on legendary. Halo waypoint on the xbox does not register my beating the game, but waypoint on the web does…HELP! I really do not want to do it again, I have like 4 achievements left!
I feel ya, man. I recently created a thread on just this issue. No one at Bungie or 343 has felt it necessary to fix this in the 3.5 years Reach has been out and it irritates me. I’ve had my campaign reset four times already.
As for missing achievements, the only thing I can recommend is keep on trying. There are some achievements my wife simply cannot unlock on her account, yet they are on mine. Like the one where you have to air assassinate the Elite from a long drop? Completed on mine, won’t unlock on hers and I did it successfully at least a half-dozen times.
The game is definitely buggy. In a perfect world, someone at 343 should be asked to look into it and the campaign resets, but it doesn’t look like they care at all.