Keyboard Shortcuts for Audio Transition not working

Been a while since I've used premiere, but the last time I used it if I pressed shift + D, I would be able to add a short audio transition to remove pop sounds between clips. For some reason this no longer works, so I went onto keyboard shortcuts and checked to see if it was still the same... it was, but now it doesn't work. Ive changed it to different keys just in case, but it still doesn't work. I've got no idea what has happened!? 

Any ideas?

Cheers