If you drag an image into the canvas and have the option enabled to convert it to a smart object automatically, when you try to edit that smart object you are unable to save it to update it. Instead you get the usual save dialog and if you save in the default folder (temporary items) it doesn't actually update the smart object.
I think this is because you are placing a file type without layer support - like JPEG. The workaround is the save the file to a new location as a PSD after you've edited and choose Layer> Smart Objects>Replace Contents... and select the PSD you created to replace the JPEG file you originally placed. I've made this a Feature Request. Hit the upvote at the top of the page to add your voice.