You should check if the image styles, for image that are not rendered, have been well created. Simply copy/paste the url image (from the src attribute of the img tag) in a new tab to see if the image style exists. If yes, then you can look at your serveur configuration (time out, etc), if not debug in the snippet above to see why the image style is not well generated. I can't say anymore, I don't know your context.
You should check if the image styles, for image that are not rendered, have been well created. Simply copy/paste the url image (from the src attribute of the img tag) in a new tab to see if the image style exists. If yes, then you can look at your serveur configuration (time out, etc), if not debug in the snippet above to see why the image style is not well generated. I can't say anymore, I don't know your context.