I'm new to the hobby, but have studied many planes in detail. Unless the wings are sheeted with balsa, there is going to be dip in between the ribs. From what I've seen, this is normal due to the tightness of the covering. Since i'm new to this, I could be wrong, but this is just what I observed on other planes. Also, I have an LT-40 and when I covered it, I noticed the same thing.