Messages working for some numbers, but not for others.

Your iPhone and Mac need to be configured properly and jointly so the Mac can send SMS messages when you are sending to users who aren't on imessage (for instance Android phones).

iMessage checklist

• iPhone and Mac both logged in to same Apple ID. Try logging off and logging on if necessary.
• iPhone Settings => Messages shows iMessage as "on" (sometimes toggling this off and then on fixes some things)
• iPhone Settings => Messages shows Send & Receive showing both phone number and email (Apple ID)
• iPhone Settings => Messages shows Text Message Forwarding and your Mac is listed and shown as "on" there.
• iPhone Settings => Messages shows Send as SMS "on" and MMS Messaging and Group Messaging both "on"
• On the Mac: Messages => Preferences shows same and correct Apple ID and "You can be reached for messages at" indicates both phone number and Apple ID (email), and "start new conversations from" indicates the phone number.

Maybe those numbers do not use an iPhone. Your phone will send as a normal text message but your Macs can only send iMessages. That is just my guess. Hopefully it helps!