I have several feeds setup in the Feed Aggregator ( admin/config/services/aggregator ) in Drupal 7.

I know that my cron job is running regularly - by checking admin/reports/dblog

But the when I look at the feeds list, the "LAST UPDATE" for each of them shows dates from over a week ago. If I do the update manually, i.e. click update items then the feed is refreshed and new items are available.

But why is the cron job not triggering the updates?

If you can't provide a reason then what approaches to debugging can I use to find and solve the root cause?

I have searched this site already and not found an existing question about this problem. I did find RSS feed not updating using feeds module but that question is about the Feeds module - which I don't use.