It seems since a recent update you can no longer unlock any achievements for the iOS version of Minecraft pocket edition after meeting the requirements, uninstalled the game, waited a few days and reinstalled. Same issue constantly occurs to me and many other people
MCPE-16617
Achievements no longer unlocking
Resolved
Duplicate
0
Daniel Tate
(Unassigned)
Unconfirmed
Phone - iOS - iPhone 6
ios 9
0.15.6
Retrieved
Thank you for your report. This issue was, however, submitted earlier as MCPE-10840. Please perform a search next time to make sure your bug hasn't already been reported. The parent issue can easily be found by searching for "mcpe unresolved achievements".