Turn the bl**dy lights on
I get how what is not seen can be scarier than what a film chooses to show you - the power of the imagination is so strong, I've literally been unable to watch some scenes in certai films because they played with my inner fears so effectively.
But there is a line that can be crossed where what is hidden becomes so obscure it has no impact at all. In this case, the whole film was made so gloomy, that the hints and suggestions of something dark, something terrible in the house were lost in the shadows. I literally couldn't see anything to even suggest I should be scared, let alone feel the fear. There were a couple of moments when the set up made the hairs on my neck stand up, but then that was lost as the scene develops and I'm just left thinking 'What? What are you trying to do here?'.
It feels like this should have been more effective, but I was actally really quite bored in the end.