A WebmasterWorld thread has several complaints from users that their Google Alerts notifications have really slowed down. Some users are reporting not receiving any alerts in a month or so.
The complaints caught the eye of a Google representative, Marcel the Product Manager of Google Alerts. Marcel explained that Google has modified the Google Alerts algorithm to be more quality focused. Let me quote him:
Regarding the volume of alerts, we've been tweaking Alerts from a quality point of view to try and avoid sending things that people aren't interested in. If you want more results, change the "Email length" setting for your alerts from "up to 20 results" to "up to 50 results" - you'll get more results with looser filtering.
If you have been getting fewer alerts, it is not a bug, it is a quality change. If you are unhappy, you have two options. (1) Try setting for your alerts from "up to 20 results" to "up to 50 results" as illustrated in the image above. (2) Complain in the forum louder.
Forum discussion at WebmasterWorld.