It usually does like a little lower light but not no light. If you want lower light can you raise your fixture?

Coraline is weird stuff. It grows and then flakes off at different times depending on tank parameters etc. Many belive that adding Strontium helps it grow. Are you adding Magnesium?

I'd focus on getting your tank running well and you'll get coraline eventually. It can also take quite a while in a new tank.

TT