Why Won't That Achievement Pop?

One achievement left on Metro 2033. Tried three times to finish Sherlock and no go! Used three different guides which all had the same routine for picking up MGBs in the stations. Followed exactly each time, made sure I had the items "picked up and in inventory". Yes, I cleared the cache after the first failure, reloaded and updated the game to my hard drive on the Xbox 360. started new from scratch runs, etc. Hope it isn't glitched but that's what it's lookin like.

I could be playing Thursday night Halo or Borderlands 2!  Any help is appreciated. TIA