How many Macs can you have text forwarding in Messages?
I have two computers and it seems to only work on one.
MacBook Pro (2020 and later)
You can have a maximum of 5 computers associated with an Apple ID and as such a maximum of 5 that will appear in Settings->Messages->Text Message Forwarding on your iPhone to forward to.
Make sure both Macs are turned on there for Text Message Forwarding.