Why aren't my Gems syncing between my devices?
Angry Birds 2 Guide
The sync issue is a well-known problem at the Roxio customer service desk and the fact is the problem was implemented intentionally as an anti-gem-stealing mechanic. The company has plans to issue a patch to clean up the sync process, but as of right now there is not really a way to have all of your Gems in any given basket, so to speak.
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