I noticed a few months ago, after not practicing the trick for a while, that when I told Wall-e to "Scoot" (back up) he offered other behaviours, but not the backing up behaviour. The Scoot cue needs a bit more work. I haven't consistently worked on the trick since then, but I did just a few minutes ago. Seems that his understanding of the Scoot cue is more rusty than his understanding of the actual behaviour, so I simply started clicking him for offering the behaviour on his own (first just one little step backward, then a stride, then a few strides). Backing up is a behaviour he regularly offers during shaping sessions (which is how I taught him originally), but I guess he just doesn't fully understand the cue well enough to respond to it consistently. More training! :) That's fine though, I love trick training!