Minecraft- Why do my Item Frames Disappear?

So recently on a semi survival server, I build a shop in survival mode. I put up lots of item frames to display the item being sold. When I go back to my home and come back, some of my item frames have disappeared. Nobody could destroy them, because my shop is still in construction and closed. If anybody knows why and how to fix the problem, please help! Thank you.

Read more: Minecraft- Why do my Item Frames Disappear?