Hi,
This has probably been discussed before, but as most of you know, most types of slabs doesn't show up on the livemap, as seen in this picture:
Before you go and lock this and link me to a previous post about this issue; I know a fix.
Update minecraft overviewer. That's it. I used a previous older version of Minecraft Overviewer and sure enough, the slabs didn't show up. I updated, and voila, they were there.
Thanks, LIEKABOWSE