Hm, I have no idea, what could be wrong. Is it maybe window 2 instead? I had a plugin open in Photoshop when testing again now, which made my document window window 2, since the plugin had a floating window which was Window 1 now. I can only test Photoshop CS 6 over here, so I don’t know what’s different with CC 2015.
However, how I found about the AppleScript in the first place was by watching this video tutorial about using KM together with UI Browser and then using the test version of UI Browser to figure out the window. Maybe you could also download the test version of UI Browser and figure out which window you need to target in AppleScript with CC2015.