go to your account settings > Email settings and make sure there checked “Item update notifications” checkbox.
Also you can contact purchased item author and let them know that they are not receiving update notification email. Because when author checked the box ‘‘Send a notification to buyers when this update is approved’’ at the time of update submission and any update version will release then customer will receive update notification email.
I am only notifying of a very big changes to the theme (eg. Ghost v2 =>
v3). Otherwise people think he needs to install a newer version which
causes a lot of problems. I do a lot of small updates so it would be
just unnecessary confusion.