– Perhaps the group texting feature on your iPhone is disabled. – Your iPhone's memory space is insufficient to receive messages.
If you've been using the Messages app for a while, it may have developed some bugs.
Almost every phone operating system has a few flaws, and the iPhone is no exception. If there is a problem with the software on your iPhone, you will experience this issue.
To access the Messages app settings screen, open the Settings app on your iPhone and tap on Messages.
– Open the Settings app on your iPhone and select General, then Storage & iCloud Usage. You will be able to see how much memory space is available