There are a few reasons this might be happening.
It could be because you don't have a personal senderId. If this is the case apply for a SenderId here :
If you already have a Sender ID/Alphanumeric make sure you have defined it in your code in the 'from' parameter.
Ensure that the Sender Id is raised in all the countries and with all the Telcos you intend to send messages to.
Ensure there is no typo in the Sender Id it should be used exactly the way it was raised.
If none of the above resolved the issue reach out to us on email support@africastalking.com