I'm running Lightroom CC classic, when I'm in develope and want to jump over to Photoshop or any of my 3rd party apps. the are showing but are grayed out so that I can not move my edits to them. I'm running a PC windows 10.
If there's any chance you're working with smart previews with the master images not connected, that could be the reason. The master images must be accessible in order to be able to edit in Photoshop or any 3rd party apps.