{wot_tracking_url} link is not always added to email

Hi,
The {wot_tracking_url} link is not always added, first it started happening that the link wasn’t being added at all when using the “send email to customer if trcking info changes” option, the one that sends the email as soon as you enter the tracking number on the single order page. The email was sent but with no {wot_tracking_url} at all, like the following:
Hi Mary,
Your order has been shipped. You can click on the link below to follow the progress of your package:
(this is where the tracking url should be but line appears empty zero url or else)
Order details:
Order #16346
…
So I discovered that if I disabled the auto-send and clicked on “send tracking email” on the orders list page then the tracking url was sent ok along with the email, so I’ve been doing it like this for a couple of months.
But today it failed even by sending it from the orders list “send tracking email”, and I had to send the same email a couple times until the url was included (the customer received the same email until I saw the url included) which is awful, extremely unprofessional and cheap.
Right now I sent twice this:
Hi Mary,
Your order has been shipped. You can click on the link below to follow the progress of your package:
(this is where the tracking url should be)
Order details:
Order #16346
…
Until the third time it was sent as it should be, like this:
Hi Mary,
Your order has been shipped. You can click on the link below to follow the progress of your package:
https://tools.usps.com/go/TrackConfirmAction_input?qtc_tLabels1=9405509898642888219526
Order details:
Order #16346
…
The Email is sent using Email Template Customizer for WooCommerce 1.1.4

I’m sure you understand how important is for this to work correctly.

Regards,