This could be a number of things ranging from misspelled email addresses to session timeouts here are a few of the most common
1. Have your client add email@example.com to there contacts list
2. Session ID Lost: If Client is requesting a download from a gallery that had been open on the desktop for a while have them close the browser tab of the gallery and reopen the gallery and request the download promptly after opening. We have seen that in some cases the gallery session ID is lost after a gallery tab is left open for some time (we believe the browser does this for security reasons).
3. Plan ahead:) Tick the CC me on download box and we will email you a link of each download request. Take a peak at the bottom of the gallery share page it will display all the active download links you can grab one of those and share with your client through your own email program.