Hi there, my name is*****'s something to try.
Place the contact group as the recipient of your message. Next to the contact group, there is a + sign. Click the + sign and Outlook will expand the contact group so it becomes a message to multiple people instead of a message to the contact group. You should be able to send it that way.
If that doesn't work, try recreating the message and sending it according to the instructions above.
If I've answered your question and resolved your issue, can you please accept the answer and rate me accordingly? I don't get paid anything for helping you unless you accept the answer and rate me at least 3 stars. If not, please chat me back and I'll get you the information you need to resolve your issue. Thanks and have a great day!