I believe the reasoning is due to the following I found in the Help documentation:
Only available when working with a QlikView Server document. Creates a mail with a URL link to the current server document. A temporary server bookmark will be created (including layout state) and encoded in the URL. The mail recipient will be able to use the URL link to open the server document and see what you see, obviously provided that he or she has access rights to the document and data therein. A mail client has to be configured on the computer for this command to work. The QlikView Server must be configured to allow server bookmarks
The key thing above being the 'temporary' server bookmark, I expect what happened is the temporary bookmark was cleared by the QlikView Server by the time the users that received the email clicked to use it. I believe the default value for temporary bookmarks is 24 hours if I recall correctly. Found the following article in the knowledgebase which indicates 7 days on those, so apologies I do not know for sure, but here is the article, and there is a link to another there that should allow you to check to see what is configured on the QlikView Server I believe. Hopefully this helps.
Bookmarks created by Mail with Bookmark as a link...
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