Hey keph427,
Sounds like you never received some group iMessages, and looking to prevent this from occurring in the future. We certainly wouldn't wanna miss out on any of the conversation either, so we'll be happy to help.
To get started, can you answer a few questions for us to help us better understand what you're experiencing:Is this the only device not receiving these messages? Do you have other devices set up to receive these messages by chance, if so what are they and are the missing messages shown there? What network are you using when you notice this occurring? Cellular or a particular Wi-Fi network?Are you having any issues sending a group iMessage?
Next, are you syncing your messages with iCloud by chance? You'll find more information on doing so here: Use Messages in iCloud
While there's not a way to retrieve messages you never received, the above questions might help us better understand what could be causing this to occur.
If this behavior is isolated only to these group messages, it might also be a good idea to start a new group message thread as well and test further.
Let us know what you find out and whether the suggestion above helps out.
Take care!