It appears that this is operator error. I apparently wasn't looking down enough in my other attempts. Apologies for the waste of effort. Please consider this issue resolved.

@Leopard20 you are probably correct in describing the issue. I am doing as you indicated but just can't get the action to activate. I will give it a few more attempts but this action doesn't seem to work as easily as the others in the game.

Once at the top of the ladder that leads to a platform or roof, it appears that the open area that you need to climb down appears to revert to a floor and you cannot take an action to go down the ladder.