My messages won’t deliver to one specific contact message to one person on imessage or text message. Same with calls.

Why is it that when i try to send a message on imessage to one particular person that also has an iphone the messages don’t deliver, and doesn’t show a failed to deliver message next to the message? Whether it’s imessage or a text message. This problem happens randomly and can go on for days without delivering a message and suddenly goes back to normal. There’s no pattern and it happens with calls too.