I can't make slabs in minecraft?

I can't make slabs in minecraft i tried it with oak wood planks but i can't. I put three oak wood planks in horizontally so it should make it. It works with cobblestone, but not with oak wood planks. I noticed that after i turned to /gamemode 2 once my acacia planks were replaced with oak wood planks and acacia stair were missing.

If you are using version 1.7.2 it may be bugs because the version's new, if not, try reinstalling the game and if that doesn't work, contact the Mojang team for further support.