Alerts/Rules question about automated responses after a brief period of time

Alerts/Rules question about automated responses after a brief period of time

Our requirement is this:

We currently create Zoho Desk tickets upon receipt of emails to our support address.
Each ticket is automatically assigned to an Agent upon receipt.

If after 20 minutes of creation the Agent has not yet replied to a ticket, we want to send out an automated email assuring the customer that the ticket is assigned to a named Agent, and that it is being actively worked on by a human, and that they will receive an update as soon as possible.

In order to do this I created a new Email Template and accompanying Alert, and created a Rule in our Tickets Module.

It is set to:

* Execute upon Creation
* when the Criterion of 'Time to Respond' is Greater Than 20 Minutes
* perform the Action of sending the Alert in question

The Alert is:

* our custom email template called 'Your ticket is being worked on'
* which sends an Email Alert to 'Record Creator' and 'Notify Contact'

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I have tested this and I am not receiving any email after 20 minutes.

Is there something I am doing wrongly?

Many thanks.