Sometimes---could be a number of reasons why it happens.
If you get the first one and then a little while later the model is back online, they don't send a notification. Don't know how much time has to pass before they send another one.
Click the model's bio and unmark the notification icon; then re-click it. (This is what Customer Service recommended when I contacted them a while back)
The notification email is on your safe senders list-right?
As a last resort, try sending them to a different email address and see if that works.
If all the above has failed, contact customer service either by phone or by email.
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