As the title says, elevators do not work if there is anything except air space between 2 elevators. I was attempting to put in an elevator system on my roof and 1 in my main floor of my base, with a single floor inbetween them. The setup would have been Elevator, 3 air space, sandstone, 3 air space, elevator. In that setup the elevators failed to let me go up or down, however as soon as I removed the sandstone, I was able to go up and down freely.

I honestly do not remember this being an issue with 1.6.4, however I might just be wishing it worked in a way that it is not ment to work.

I would check your config file to see if the elevator setting is not at the default of 4 blocks. I personally like it set to -1 which ignores blocks. If it is on a server check with an admin/mod to find out what they have the server file set to. If they have it set that not even one block can allow the teleport and they are not willing to change it, then there will be little you can do.

I hope that helps you, if not, someone from the team should respond soon.