One person can’t receive my iMessage

If you have enabled iMessages and sending a message to another iPhone user then it will be always sent as iMessage. The same will be sent as SMS only if...

1. You have no internet either over WiFi or Cellular
2. The recipient has not activated iMessage.

Types of messages?

• iMessage? --> Check your internet stability and connectivity. If the Apple server is the issue, it will show here --> Support - System Status - Apple
• SMS/MMS? --> Does not need internet but contact your Carrier Service Provider for network coverage and outages ->: Down Detector - outages

Are you receiving his messages from Apple ID or Phone number?

He may not have iMessage enabled for his Phone number and also may not have enabled SMS plan for his SIM through his carrier.

In that case he can send you iMessage through his Apple ID but you cannot send a text message to his phone number. He also cannot send you an SMS from his phone number.

Try sending an iMessage to his Apple ID.