I notice that sometimes a new page with a keyword that I have an alert set for will trigger a Google alert, and sometimes not. Mostly not. My Alerts are set for "all,everything,etc". This occurs when guest blogging here and there. The blogs are all legit, real traffic, & all of the pages show in the cache within 1-2 days of the post, sometimes same day. Alert keywords are always in page title,and/or url, etc.
It seems that somehow new pages and/or new content must meet a certain quality signal or other metric to trigger an alert. Have others experienced the same? Does anyone know more about it?