Scheduled tasks not running

Hi,

I have 12 separate schedule tasks that is set to run monthly on calendar days 1-15. It runs every 2 hours starting from 12AM. (Next is 2AM, 4AM, and so on)

I am encountering inconsistent issues with 6PM and 12AM where it does not kick off at all on random days. I've checked the configurations and can confirm it is set up right. 

Any idea what's causing the inconsistent failure to run? I'm in central time if that helps.