btbuses The issue is random? It generally means that you are getting some errors from your server / PHP when attempting to generate resized images (thumbnails). If it's random, it would mean that your server is struggling with sufficient memory when handling multiple resize requests. This would be fairly unusual, unless the images are extremely large > 5000 px, but the ones in your screenshot is small.
You could diagnose the exact error response by opening one of the resize image requested in new tab. That could be achieved by going to browser developer tools > network, and then refreshing page.
What server is this? It is not a common issue, but yes it could happen for various server related reasons. You should try to resolve the issue, although in the case it's related to memory limits on your server, it's likely you would be able to create the thumbnails after refreshing the page.
