i have created email alert for the task module, some things it works, sometimes specially when due date has edited it dose not work?

i have created email alert for the task module, some things it works, sometimes specially when due date has edited it dose not work?

1) Basic information -  the rule is active.
2) Execute on - create or edit
3) Rule criteria - 
1   Due Date is ${TODAY}
4) Action - send alert