First: Reduced accuracy doesn't mean poorer play. In unclear positions accuracy will be low and in positions with a clear plan accuracy will be high.
Second: In closed positions it is important to understand concepts like open lines (and why doubled pawns can be an asset), pawn breaks, influence in the center, why the Knights can be superior of the Bishops, wich endgames are better etc. It is hard to learn without a book. I will recommend Nimzowitsch's My System.
I've noticed that when I'm in closed positions, my accuracy reduces considerably. How can I improve?