Hi rose- I remember a couple old mansions where that happens. The only way to pass is to go back up the ladder and re-enter so eventually Jack lands on the right spot. Also try lowering your monitor resolution (so the playing screen is bigger) and slowly creeping down the ladder.

In the past, changing the ladders would have meant having the designer fix it (or giving permission for someone else to do it.) Not sure at this point though, since a lot of designers have vanished and Vern isn't supporting MM anymore.