Change to wither spawning. With Wither Skelly skulls, you can spawn more wither skellies, but to spawn the Wither directly from a snowball requires 4x4 powering, namely two Nether Stars. Therefore you have to already kill two Withers to be able to spawn them from snowballs, and two glowstone blocks won't be enough. This is consistent with the 'to make a thing you must already have access to it normally' that we have with Milk Bucket.