Phone - Android - Xperia Play
I found this glitch lately, that makes you unable to place trapdoor on a solid block if there's an unsolid block at the back of it. You then have to replace the unsolid block with the solid block, place the trapdoor, and replace the solid block with the unsolid block you want to have there (as seen on the pictures).
Pic 3: Can't place the trapdoor on the block I want.
Pic 2: Replaced the glass with sandstone, placed the trapdoor.
Pic 1: Replaced the sandstone with glass.
I've tested it in PC Minecraft, you can do this.