Ships as well as planets, asteroids and stations are made of 16x16x16 chunks. If my theory is right, symmetry plane is bugged and can\'t at the moment create new chunks, or if it can, it works a little \"randomly\". So you have to create the data chunks first before you start detailing. 1 long line in all directions should generate them. Basically create the ships bounding box before relying on symmetry-plane.
I\'m usually starting with this kind of pattern:
+----------+----------C----------+----------+ (10 blocks, marker, 10 blocks, marker etc)
This lets you know that measures are right on both sides. If other side falls short, continue building from there instead.