When food or toys are not reinforcing....
About eighteen months ago, I found myself at my front door, my two shelties were offering a lovely sit, waiting to be released to go outside. My Koolie x Kelpie puppy, Tom, who was about five months old at the time, stood there blankly staring at my front door. So I asked him to sit again, told the shelties how great they were, and Tom continued to stare at the door and then came the moment that has changed how I think about reinforcement forever.
I offered him food.