Email alerts are low priorities tasks in Marketo.
The reason for this is that some other, higher priority tasks, have been jamming your instance the whole day.
So you need to look into what's overloading your instance and try to work on this.
Without more info, hard to give you more help
My assumption is campaign is recurring batch type which is set to run every day at 10pm. Can you please share screenshot or explain structure of your campaign?