1. If you click on the Approve button and see a window with the message 'Updated', it means that the request has been updated after the notification was sent. The solution is to close the notification window, refresh the browser window, check the updated request and then make a decision.
2. If there is an error message saying the document can not be found, it means that either the document was deleted or you've been removed from the list of Approvers.
3. If you are not able to click on the button, there could be a problem with an email client: http://support.approvalmax.com/knowledgebase/articles/1131817-why-buttons-in-the-email-do-not-work