When my Magento 1.9.3 install rebuilds the cache after an image flush, not all images are included for some reason. The images are there and have uploaded successfully, but are not being built into the cache so they don't show up on the frontend or backend.
-set memory to 2048M
-made sure images were set to "global"
-checked permissions on the server
-cleared cache, image cache, and re-indexed