An editor is not able to tag (@) another editor shared on a proofing system, what could be the cause

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An editor is unable to tag another editor that is already shared on the proofing system, this has been happening quiet a few times, this editor ends up having to type out the email address and it is sometimes incorrect then queries are left hanging or does not reach the correct contact. What could be cause and how can we go about correcting this?

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