Hello, annika179!
Thanks for using Apple Support Communities! From your post, it sounds like your notifications for a group chat are not being delivered. We don't want you to miss out on any important conversations.
It's possible this chat has been muted. To check, open the Messages app, then swipe left on the group message. If it says "Show Alerts" this means the conversation was muted.
If it has not been muted, try deleting the group chat, then starting a new conversation.
Should the above steps not work, you can check the notification settings by following the steps below from this resource: Use notifications on your iPhone, iPad, and iPod touch - Apple Support
"If you don't see notifications for a specific app
To get notifications, connect to a Wi-Fi or cellular network. If you still don't get notifications for a specific app, try these steps:
- Make sure that the app supports notifications. Go to Settings > Notifications, select the app, and make sure that Allow Notifications is on.
- If you have notifications turned on for an app but you're not receiving alerts, you might not have Banners selected. Go to Settings > Notifications, select the app, then select Banners.
- Make sure that you’re signed in to your Apple ID.
- Make sure that Do Not Disturb is off.
- If you’ve recently installed an app or restored from a backup, open the app to start getting notifications."
Feel free to post an update for continued support.
Take care!