I have this same problem and I'm with Optus. Nokia can you please let us know how we can fix this??
I take it you've been into the messaging settings > Advanced and told it to send a group MMS as opposed to an SMS to all members of the group?
I wonder if it's a country restriction as my messaging app is set to UK so that may be why I have the Advanced settings.
MMS has always been unreliable on any android phone i have owned, thank the lord for Whatsapp that's all i can say given the UK network charges for MMS are now stupid high.