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
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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".