Can't place slabs of different types onto each other

What I expected to happen was...:
I would like to place 2 type of slabs (for example stone slab and nether brick slab) onto each other, sometimes it works, the other time i can't

What actually happened was...:
Sometimes i can place them onto each other, but then if i go 1 half higher, i can't

Steps to Reproduce:
just place slabs of different types onto each other