What are you using exactly to measure humidity? The round analog humidity monitors don't work at all and are super inaccurate.
^ The above advice with the shoebox only applies if you're certain the snake officially has a retained shed, and isn't simply in the middle of trying to shed. If your humidity is at least 70% (measured accurately), the snake should have no problem finishing the shed on its own. Additional misting really isn't necessary unless you're having a problem with dropping humidity.
Be sure your substrate/ground of the tank isn't wet, or it could cause scale rot.
ETA: I just read that 24 hours passed with the shed, it's definitely a retained shed!