Thank you for your suggetions. I found the drawing and somehow I had completely overlooked it. Then for the really embarassing part, I discovered that the red rectangle on the floor was the pressure plate. I thought it was under the floor by the switch and couldn't be seen. So when I finally came to my senses, I drew the correct symbol on the slate and left it on the real pressure plate and everything worked! Thanks again and thanks for your patience. Now onto what looks like a maze....

... for which you should already have found a pictographic solution.

I have been experiencing the same problem with the lift but reading through this thread certainly switched on a light bulb. I realised I'd been using the wrong Yeesha aymbol (circle on top of stick with left facing right angle at bottom)from Journal 10 (I think) instead of the one from her seventh journal which also had the correct slate shape (thanks Mr Zebe D). By the time I'd finally come to this conclusion I felt just like that symbol looked - when you find it you'll know!
Thanks to all for the hints, now onto the maze and colours before my eyes.
JB

I'm confused - I've already made 2 elevators work and never used the slates. What am I missing here - please someone tell me?????