I have looked into this issue and have deduced the cause. It appears that anyone who had completed all of the “Dragon’s Reach, part 2” achievements before the addition of “Dragon’s Reach, Part 2 Mastery” cannot purchase the ascended nomad amulets “Aspect Amulet” and “Ogden’s Ankh.” The glitch is likely due to the requirement of having “Dragon’s Reach, Part 2 Mastery” achievement activate upon completion. This does not happen when all achievements are completed before the mastery achievements were added to the game.
Arena Net I highly encourage you to resolve this issue as it is frustrating and unfair for my fellow players who have worked hard for their rewards.