When you stand on a bottom slab, if you have a bottom slab one block above the one you are standing on and one block in front, auto jump doesn't realise that you can jump and doesn't. To recreate, place a bottom slab on the ground. Then in front of it, place a top slab and a bottom slab on top. Turn on auto jump, stand on the lower bottom slab, walk forward into the bottom slab on top of the top slab and auto jump won't jump.