You may meet the following error when trying to run a cron in Drupal

• Attempting to re-run cron while it is already running.

Or the cron was completed immediately after u input the cron.php url in browser.

When you get the above situations, that means your cron fails. To resolve the issue, delete the following to record in your Drupal database.

• delete from variable where name = “cron_last”;
• delete from variable where name = “cron_semaphore”;

Then clear the Drupal cache and your cron should be able to run now.

Done =)

Reference: Midwestern Mac, LLC – Cron Troubles in Drupal? Here’s a Tip