If your scheduled campaign continues to fail getting processed you probably have too many active time-driven triggers.
To clean them up, please proceed as follows:
1. Unschedule your campaign(s)
2. On your spreadsheet menu go to: Tools > Script editor. In the new window that appears click on Edit > All your triggers. Click on the little crosses and delete them all.
New trigger(s) will be installed when you re-schedule your campaign(s)