Group texts won't send

Sending messages in group message already notifies a lot of people so I can see why you don’t want to send multiple shorter messages and cause phones to ring off the hook. lol 

The size limitation for outgoing messages is 1MB. A picture could easily be over that, but I highly doubt two paragraphs of text would get you there, so I doubt that size is the issue here. Since the conversations are ongoing, we know it is not an issue with the phone number or message being blocked by the receiver(s). Good to know that the data is working so we know it is not the multimedia aspect of the message causing the hiccup. I suggest these things from here:

• Make sure that the phone software and all the apps you are using are fully up to date.
• Make sure that you have at least 10% available memory space available on your phone. Once space fills up, everything can start to act funny in different ways. 
• Delete old text messages. AS MANY AS YOU CAN. I especially would start with the conversation threads that are having the most issues (delete the conversations completely if you can). Even if you have plenty of available space on your device, when you have too many messages on your phone, sending and receiving can be unreliable and frustrating. I know that a lot of information in texts can be important to save, so explore the different ways to back up the info before you delete it. You can take screenshots and back those up to a cloud, use a third party SMS backup app, or some other idea that I may not even know about. You mention a few different messaging apps, so I encourage you to delete messages from those as well. 

Deleting messages can be stressful and time consuming, but in my many years working tech support here at T-Mobile, I have seen it work more times than any other tricks I have tried. 

I hope this helps.