How to solve problem if your Drupal cron fails
Sometimes I have problem with Drupal cron. There is module which solved my problems.
When I tried from Reports page in my Drupal site run cron, it fails.
Then I installed this module: Ultimate Cron
You can download Ultimate Cron module from this link: download
Problem for example appears when I add Flat Rate shipping for my Drupal Ubercart module.
I tried to run database update and also clear cachce from: admin/config/development/performance but nothing helps.
After enabling Ultimate Cron module my problems dissapears.
Maybe this module will helps you also when you get stack.