Photos won’t download

Recently, the photos on my phone have been taking a few moments to “load” and clear up when I tap on them. However, more recently, some pictures just aren’t loading at all. I’ve tried many different times with different WiFi connections and nothing seems to help. It won’t even let me text them out to people are edit them because they aren’t fully downloaded. I’m not sure what this means and what caused it, but I would love to know how to fix it! These pictures are super important to me!:)