Goblin Invasions are besides the requirements randomly triggered, you could wait for a long time.
The requirements are pretty much as you already state:
- One player in the game must have 200+ current life.
- There must not have been an invasion within 7 in-game days.
- At least 1 Shadow Orb must have been destroyed.
Further the wiki links to a forum discussion that states:
It triggers at the very start of day.
See Source for the entire post
I would love to include his C# converted source code in here as well, but the formatting goes bananas
Also as Yves wrote, you could also farm the Scouts to make a Goblin Battle Standard
Using 10 Tattered Cloth, dropped by Goblin Scouts and 5 Wood, at a Loom.