My Magento cronjobs are not executed properly

By default the Magento application settings are such that when cron.php
is being processed for a period longer than 15 minutes after a job was
supposed to be completed, it is marked as missed and is ignored. This
setting is under System > Configuration > System > Cron > Missed If Not Run Within.

That is why you need to change this setting to a value greater than 30
minutes. For example you can set it to 35 minutes. Otherwise tasks that
are only scheduled once a day or so will get marked as missed.

After that set Schedule Ahead for to 35 and the cronjobs issue should be resolved. You can check the Magento cron log by visiting:

http://YOURDOMAIN.COM/cronmon.php