By default Expiration Reminder doesn't send notifications after an item has expired. It will send one last notification the day after the expiration date.
You can turn on notifications after the expiration date at the category level.
Sending notifications after the expiration date
To enable sending notifications after the expiration date, first click on Your Name on the top-right menu and then on Settings. From you, you may click on Categories on the left menu.
Then click on any category from the list to edit it or create a new category.
In the edit screen, from the Category tab, scroll down until you find the Send alerts after expiration section. Turn this option to Yes and view the details below - you may choose whichever frequency you need for the category.
Every: this indicates how often or how spaced apart notifications will be sent. In the screenshot below, they will be sent every 7 days (every week).
Indicate how many after expiration alerts to send: this indicates how many to send. In the screenshot below, a total of 12 notifications will sent meaning 12 notifications every 7 days which will be one notification per week for 3 months. Keep in mind this value can't be larger than 99.
Then click Save.