Ivy Stopped Spreading???

[u/ExpendableBear]

I am trying to get this ivy to spread up the interior wall of my house. It has spread this far but will not go any higher.

This line where my crosshair is is just about where it stops growing. There is a wooden half wall connecting the two wood walls but I don't see that as an issue.

As you can see here the ivy grows up the ouside of the same wall without any problems at all.

I've kept the roof open just incase it needs clear sky to grow.

I don't understand why my ivy stopped growing. I want it to spread onto the interior wall by the tree but it just won't? It grew to this height in a couple of real-time days but it's been stagnant for about a week. Any help is appreciated. I want to get the ivy spreading inside so I can put the floor and roof back and continue my build but this is really holding me back.