Email notifications are not reliable/not working

Hi,

I am in the process of getting a Build ready to go live with, but I cannot manage to fix our email notification issues. 

1. Creation and Resolution Emails do not always send
2. Notify Assignee & Notify Customer attributes has not produced any notifications for Requests.
3. Notify Assignee & Notify Customer attributes send notifications but not consistantly - succuessful email notifciations are less frequent compared to Creation and Resolution emails.

If you need any further information please let me know I can provide it.