If you change messaging apps on your phone and don’t receive messages, you may need to turn off chat features.
Delete the contact and add it again.
Check if you blocked them.
Verify that their phone number is correct.
Check if you need to specify a country code (e.g. +1 for US numbers).
Ensure that you have a data connection.
In your device’s settings app, reset your APN settings to default.
If you get a notification that Messages may not send or receive messages until more space is available, try these tips to free space.
If you switched from a phone with visual voicemail to one without, and you get unreadable texts from your mobile carrier, troubleshoot your voicemail notifications.
If you're switching from an iPhone to an Android phone, be sure to turn off iMessage.