There is a large Google Web Search Help thread with complaints that Google Alerts stopped sending users their notifications.

It was a bug that surfaced around January 10th and was supposedly resolved the same day.

Top contributor, bluequoll said:

But shortly after people said it was fixed, there are still some users saying it doesn’t seem to be fully resolved.

Are you having issues with Google Alerts you subscribed to?

Forum discussion at Google Web Search Help.