This sounds similar but not identical to what I'm seeing with my phone. My phone has problems with multiple-recipient messages. I can view and reply to the messages fine, but despite that I still get error notices about "message not downloaded" and the app's icon on my home screen will report an ever-increasing number of unread message. This continues, as best I can surmise, until I delete the message thread and power cycle the phone.
For me, this only happens with multi-recipient messages, one-on-one messages are always ok.
And also this only occurred after Motorola pushed the KitKat virus to my phone. I never had any problem with multi-recipient txt's before that.