They are very good quality panes of glass to be fair, thank the Swiss builders and their excellent insulation! After advice here I moved the tree to the south facing side of my apartment, that does get sun. However, the problem with this side is that the window has an enclosed balcony in front of it, so even though it gets some sun in the afternoon, on average I suspect the light is less than my north facing window, as there's less sky visible, if that makes sense!
I had him at the north window since I moved here in December, and I was shocked at how much growth it was getting in December/January in a north window that gets no direct sun. It's a really big window and has full view of the sky. I've moved it back there for now..
I think you're right and it is light it needs, but without a grow light I am at the mercy of the time of year