Regular SMS are sent between end-users' phones, such as when you send a message to a friend or a family member. These SMS are sent via the Person-to-Person (P2P) network, whereas SMS from MessageBird are sent via the Application-to-Person (A2P) network.
Because there are different networks involved, a phone can lose its connection to one network but stay connected to the other. As such, it is possible that a phone receives messages via the P2P network, but will not receive any messages via the A2P network which were sent at the same time.
A restart of the device usually resolves the issue, as this will force the phone to reconnect to the A2P network. Alternatively, the phone can also be put into Flight Mode for about 20 seconds. When you turn off Flight Mode again, the phone usually reconnects to the A2P network.