There appear to be several bugs surrounding the EoW. Everything ranging from his head appearing separately, to him only attacking players who aren't in corruption...you name it.
I found several forum threads, the most recent being this one (From Thursday, January 5th) with 2 people describing 2 different bugs.
The OP of that thread mentions still getting the message though, which prompted someone else to say
if the message appears, it is not the same bug as the multiplayer bug. the multiplayer summoning bug only introduces the noise. if you get the message - most likely he spawned and something made him despawn or you are experiencing a new type of bug.
We don't get the message, but AFAICT do get the noise, so it looks to me like we may be encountering "the multiplayer summoning bug".
Additionally, James had a very good run spawning him over and over again in both singleplayer and multiplayer, so it would appear that if there is a cooldown timer, it's too small to be noticable, and definitely not what's affecting us now.