Few things are quite as frustrating to an achievement hunter than meeting the requirements for an achievement, and having it not unlock. All too often, when it happens, there is very little you can do about it. You have to hope that the developers care enough to be bothered to fix it, or that someone, somewhere, happens to find something that you can do to trick the game code into re-analyzing the achievement's criteria and make another attempt at popping it.
I've had this issue with two games very recently. One, a simple city-building game for the Windows Phone called Kingdoms and Lords, actually has several achievements that may or may not unlock. You must build armies and attack certain "bosses" in the game, and the defeat of each one unlocks an achievement. Usually. Any one of the boss achievements could completely fail to unlock. If you do not know this ahead of time and are not paying attention, you could very easily move on to the next task and save your game at a point after the boss is defeated. Because that's the only way you can get the achievement to unlock — by reverting to your previous save point and replaying the final battle. While you can replay the battle at any future time, it doesn't count it as defeating the boss again, and the game will not re-evaluate the achievement and attempt to award it. The only alternative is to completely restart the game — but, since it's a typical free-to-play game that only allows a certain number of "turns" per day, it can take quite a bit of time to rebuild back to the point of the boss in question.
The other game in question is a much higher-profile game, Titanfall. This pride and joy of the Xbox One has been widely viewed as the first big reason to get a next-generation Xbox console. You can earn experience points to level-up your character to level 50, at which point you can "regenerate" and start over at level 1, doing it over again until you regenerate 10 times. There are two achievements that are closely related: Maxed Out for reaching level 50, and My Generation for regenerating and starting the next cycle. I say "closely related" because reaching level 50 is the requirement for being able to regenerate, so once you earn Maxed Out, you have the ability to hit a button, regenerate, and pop My Generation.
To fuel the fire, they have added a challenge called Early Adopter, for which you must reach level 50 in the game's first month (by the end of March 2014). Ideally, reaching level 50 within that time frame would pop both the achievement and the challenge.
Unfortunately, it doesn't seem to work out that way in practice. Some time in the last week of the month, I reached level 50, and neither the achievement nor the challenge popped. In researching this issue, I've found I'm not alone, but sometimes things can be done to have the game reevaluate the criteria and pop it. Some have said that it pops on its own after playing a few games. Some have suggested hard-rebooting the console. Still others have noticed it pop after regenerating.
I've hard-reset my console already (since the Xbox One can't seem to go a week without needing to do that for some reason or another). I also played many games since then. There is another achievement for completing all weapon challenges to unlock all accessories. Since I was very close when I hit level 50, and since regenerating resets those challenges, I decided to play until I unlocked those items first. Ultimately, I did earn that achievement — but the Maxed Out achievement and Early Adopter challenge remained locked.
I even went above and beyond the call of duty in trying to re-trigger an achievement analysis. I deleted my saved game data, deleted the game off of my hard drive, and re-installed. I was a little concerned that it would lose my level progress, but I found that I need not have worried — as soon as I started the game, it showed a message that it was syncing with the servers, and I had my level 50 and all challenge progress and item unlocks back. Unfortunately, my achievement and challenge were still locked.
It is almost insulting to pull up your achievement progress and have it say "100%" next to the word "Locked".
I played one more game at level 50, and then decided to go ahead and regenerate. The My Generation achievement unlocked like clockwork, and I was back at level 1. So now my achievement history shows that I did regenerate on March 29th, somehow, apparently, without reaching level 50.
I have read from random internet commenters that the achievement does sometimes unlock when reaching level 50 the second time around. However, I doubt that I will be able to accomplish this in the last two days of the month, and so, despite having met the requirement, the challenge will likely remain forever locked.
It's not really a big deal, since challenges are not counted with achievements and earn no gamerscore; and as long as Maxed Out does unlock when I reach level 50 again, I will still get my points and may possibly "complete" the game. But it is just generally annoying that I've done what I'm supposed to do, and I get no credit for it.