I puchased a new iMac and there is one entry in my contacts app that no longer works with Messages I recently bought a new iMac running Catalina and I used Migration Assistant to load some of my old stuff on my new computer.  This appears to have worked well and my new computer is running fine. There is a problem however. I have several contacts that I text with a few times a week. One contact however doesn't work. I always get a return notice on the messages I send. I can use this contact on my iphone and my iPad and the messages go thru. But it doesn't work using my computer. The other contacts I communicate with work fine from my computer. So I'm lost on what to do. I've tried restarting Messages and restarting my iMac. Neither helped. I also made a new contact card but that didn't help either. Does anyone have an idea of what I can try to do to fix this?  thanks.
I found the answer from bmike. It is given here and here. In words the method to solve the problem is:
From you iphone:

Open Settings.
Scroll down to Messages
Tap on 'Message Forwarding'.
Ensure that all the devices are selected.