Hello. I've tried searching everywhere, including here, but I still can't solve my problem. I made a slab class that extends to BlockSlab and overrid func_150003_a to return my slab instead of vanilla's stone or wooden slab, but It still gives me this bug. I can't place them on top of each other in the same block (making a double slab). The slab's top or bottom faces are also darkened out, depending on which half of the block the slab is. (If it's on the bottom the top face is darkened out and vice-versa).
Here's the image of my problem: http://i.imgur.com/4bRxsji.png
I have also registered both blocks (slab and double slab), using the same class and just changing the boolean in firestoneSlab(boolean b, Material m) to false or true, respectively. The blocks and textures are fine, its just that the slabs don't merge.
I really don't know what the problem is, if someone could help me I would appreciate it.