what you mean
"I think "Set isClimbing to True" on the Up Arrow events should be toggle isClimbing instead."
Scratch that, my mistake. I missed it the first time, I see now that they are both set to True.
I have a feeling that it's your animation control. Try "play from current frame" For your Down arrow animation
I attached a screen of a climbing I did a while ago. May be of some help.