events not spawning mobs in my new custom built area. Why Not?

[LaggyMrBrown]

So I built a simple arena for pumpkin moon but no mobs will spawn there, I teleport hom and they are happy to come there and own me and my npc but not the new area, same with goblin army.

Is there anything I might have done to stop them spawning?
I am not sure what spawn conditions need to be for monsters


Edit: hmmmm normal mobs seem to spawn there like slimes and the like.
Edit 2: solar event works there too, but not goblin or pumpkin moon, not tested frost

[big demon energy]

did you add walls? (aka background panels)

[LaggyMrBrown]

no walls but a few background panels as design but that was after I tested it so they didnt make a difference :(

[Tinvai]

Sometimes it can take a while until the event mobs arrive.
How long did you wait for them to arrive?
When I summon events I usually walk around a little, meaning few screens to the left/right, this (at least to me) seems to quicken the spawn.
Also, when there are underground gaps the mobs tend to spawn right underneath your planned area.

Hope this might help you.

[LaggyMrBrown]

I waited ages, then teleported home where they attacked, then I teleport back and they never came :/ there are no gaps below the arena, I filled them all in

[Tinvai]

Did you summon them in multiplayer or singleplayer game?
Because usually, event mobs only spawn where there's a player near by.