Wither won’t spawn

I just farmed enough heads and I’m trying to spawn the wither on my Minecraft server. The difficulty on the server is set to normal, not peaceful, and I made sure the last block I placed was one of the skulls on wither.

But it just won’t spawn. Any clue why this is happening and how I can get it to spawn? I really need nether stars.

The server is running Minecraft 1.14.3, if that helps. Are there any known bugs with withers? Cause this is super strange.

Also I made sure the two blocks on either side of the “tail” are air, not grass, snow layers or anything else, but it didn’t help.