Pictures sent in text messages appear as question mark in thread shortly after being sent.

There are random pictures/videos that I have sent recently and when I look at the thread they show only as a box with a question mark. I have saved most of these pictures but it's frustrating they no longer show in the text thread. Why? I recently got the new iPhone 13 and have tons of storage, have reset my network, restarted, turned imessage on and off...why is this suddenly happening? It's so frustrating!