Delayed iMessage delivery from coworker to MacBook Pro

This has been happening for a couple of months now. When my coworker sends me an imessage to my computer (Macbook Pro) I do not receive it until I message him. So a message he delivered to me is being held up until i send him a message. A lot of times he will call me and say did you not get my message. Then I have to send a message to him for it to come through to me. Why is this happening? Both of our computers are up to date with software.

[Re-Titled by Moderator]

Original Title: Imessage from my coworker does not get delivered to my laptop (Macbook Pro) until I send him a message