When teleporting from one Elevator to another any half slabs (or carpet) is ignored at the block you're standing on, however if there is a non-full block on top of the other elevator block the teleport will not commence.

I believe the cause is somewhere in the checking logic for TileEntityElevator.findLevel

It's at a different part of code where it checks that y and y+1 are air
The logic might need some tweaks

Having this too. Had to move the sign of mine.

Erm, this is fixed. Not in a proper way. But hey, I don't care. HELLO!