Here are some of the most common reasons:
If you're using an Authenticator app for your 2-step verification codes, you will not receive SMS to your phone. Please use the code from the Authenticator app to log in
Due to issues with the SMS network itself, codes may be undeliverable to some customers. To prevent this, it is recommended to use an Authenticator app for 2-step verification if possible, as they do not require internet connectivity or SMS coverage once configured
Your device's SMS inbox may be full. Please try deleting some messages from your inbox and request a code again
The phone number associated with your Voyager account is no longer in use. Please contact firstname.lastname@example.org.