Make sure that PS is set as the primary for the file extension which you are using :
To set that go to preferences>File type/Editor, select the files extension from the list and then add PS under the editor category.
If the above steps are already tried then you can try to reset the preferences:
To do so refer the following link:
@careyonadobe, To add to Harshit's response, another thing you could try doing is not to use the 'Preset' list of editors to choose 'Photoshop'. Instead, click on '+' sign as shown in my screenshot below, browse to Photoshop location, select the .exe or .app file for Photoshop from your hard drive. See if it helps.