Approval via email notifications

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I have two users who appear to have the same system access.  Both are receiving email notifications as expected

- one is able to approve via the email notification;

- the other is not - they click on the Approve button and nothing happens,  If they approve via the email and i check the transaction console i can still see the transaction is awaiting approval from them.  They then have to log in to the application and approve from within their worklist.

I've checked the users roles / access / profile options and i can't see anything that is different between the two users.  Has anyone else experienced the same or similar issue ?