There could be a variety of reasons - you should check the campaign reporting to see which error message is displayed. Once you have the error message, review what it means on this support page.
A tip is to make sure the test user's email address is only assigned to one contact record in the account. For example, if two contact records share the same email address (whicd does happen with test and admin users in particular) but one has opt-in for mobile and the other does not, the test message could be sent to the second contact, where it would not be delivered.
Bluesky
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