Have you enabled logs and checked the logs? There might be some information there about why you are having problems.
Also, Senders are required to use a pre-registered alphabetic sender ID. To request a sender ID from AWS Support, file a support request. Some countries require senders to meet specific requirements or abide by certain restrictions in order to obtain approval. In these cases, AWS Support might contact you for additional information after you submit your sender ID request. Please check the below link for more details.
Also, it would be important to check on what you see in the logs. Another best practice would be for messages that are critical or time-sensitive, such as a one-time passwords, leveraging TRANSACTIONAL message type in Amazon Pinpoint is indeed the recommendation.
Pinpoint, SMS not received (Vietnam)asked 5 months ago
Pinpoint SMS OTP: Amount of retriesasked 4 months ago
Is there a separate message type for OTP in Pinpoint. We want OTP messages to be delivered soon than transaction messages
Pinpoint Transactional SMS marked as SPAM in UKasked 6 months ago
Pinpoint Showing otp message delivered but not receivedasked 7 months ago
AWS Pinpoint is not working for some provider in Indonesiaasked 3 months ago
Receiving CARRIER_BLOCKED errors on Pinpoint SMS Serviceasked a month ago
Pinpoint SMS - International Usageasked 6 months ago
Pinpoint not receiving SMSasked 10 months ago
Does using a separate account for sending OTP messages give us a better OTP SMS delivery speed than sending from one account from which we send out transaction messages & OTP together