Most likely, it is due to increasing workload on the SMS provider server. Here are some probable explanations:

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If resources are shared by numerous customers, your SMS is placed in a queue and sent only when all prior jobs in the queue have been finished.
It could also be due to a misconfiguration of the SMS task scheduler; if one of the jobs scheduled before yours failed, the service may halt completely or enter an indefinite retry loop until the incorrect SMS is ultimately delivered.

I would recommend double-checking if the SMS job scheduler is up and operating, how the SMS service was initially designed, and whether resources are shared with other customers.