This post is to diagnose when a customer that used to receive your text messages, no longer receives your text messages. Below are the steps to troubleshoot why a customer of yours may have stopped receiving text messages, and how you can fix that.
1) See if there's any record of the customer's mobile phone number in your list, by searching all different subscriber states (subscribed, cleaned & unsubscribed). To perform a search for a customer's mobile phone number within your list, see here. You want to do this to make sure that they were subscribed in the first place, or to make sure that their phone number wasn't unsubscribed or cleaned, which would be the reason they're no longer receiving messages. If the subscriber is unsubscribed or cleaned, you'll need to have them opt-in again, by texting your list's keyword to your list's short code.
2) If you find the customer's mobile phone number and their subscriber state is "Subscribed", check their messaging history to see if Tatango is successfully sending and receiving messages to that subscriber. If you notice that the most recent messages to this specific customer are bouncing, check that the wireless carrier for the customer is correctly stored for this specific subscriber. If the wireless carrier that Tatango has is incorrect, try resetting their wireless carrier. If the wireless carrier is correct, but your messages to this specific customer are still bouncing, this could mean that they're out of text messages, outside of their coverage area, etc.