I see on other forums this is a known issue, but can't see it here under Nokia 6. Tho someone has the same issue on a 7.
If I send a msg with pic attached the MMS says sending... and no errors appear but the msg isn't sent.
I have only had this problem since the update to Oreo (amongst other Oreo issues I might add).
One suggestion has been to roll out all updates to the app and then reinstall. I tried that but no luck.
had a suggestion to try:
try MMS again - worked.