Wither boss won't spawn

I wanted to spawn the Wither boss to see how it looks on a texture pack. I have placed all the blocks as seen in the picture. It didn't work. I thought maybe it was because I built it in a desert biome (a random guess), so ten I flew over to a nearby forest biome and built one there. It still didn't work. I thought maybe it needs to be in the dark (I was pretty sure it wasn't the case, but still wanted to try it out) I went down to a cave and it didn't work. Maybe it needs to be built at night - nope, maybe the middle skull has to b the last one placed - nope, it needs to be built in a newly (1.4.5) generated world - nope, on a superflat world - nope. I even went on the MinecraftWiki to check if I was building it correctly - I was, I checked the bugs section on the MinecaftWiki page, but there was nothig of this sort. By then I decided to go a bit more technical - I have started Minecraft on the official launcher and force updated (I usually use the magical launcher (easier to deal with mods)) and I tried it in all of these places again and it still didn't work. Then I lost all hope and came here and when I found no similar bugs already reported I decided to report this bug and see what happens 🙂

P.S. Thanks for reading this longish note on the bug :3

P.P.S. <3