PeterC2: I finally found the answer to this problem. It is not a bug, but a built-in safeguard. Only one walkthrough mentioned this problem stating that if you "can't reach" the amulet, it meas you missed something important and need to back track and find it. I hope this helps others.
Bolch76: Thanks for the clarification! :)
It sounds like very poor game design. The player could easily think that the amulet is simply a red herring and leave it for good after trying to reach it multiple times. Also, it doesn't make sense - within the logic of the game - that you could suddenly reach something what you couldn't before.
Yes, well, this setup is necessary because...
***SPOILER ALERT***
picking up the amulet teleports you to a different part of the house, from which you can't backtrack and, if you don't have everything needed for the new area, you will be utterly stuck.
