Hi Andreas,
Draw a picture of your sail plan from the side view, luff,curved leech, square top head and foot. Now draw a dotted line form the forward end of say the second horrizontal batten down from the top, the very flat area of the sail, to the clew corner. This dotted line is the hinge axis. Aft of this line the sail falls off and in cross section the sail forms the "S" shape. The battens higher in the sail will have a greater percentage of their length aft of this dotted line, the hinge axis, and the fall off will be more pronounced here. The bottom batten or two down close to the foot of the sail will show very little fall off or "S" shape because such a small percentage of the batten length is aft of the hinge line. Does your sail have tapered battens in it? Are they very stiff in the aft 1/3rd of their length and soft in the front 1/3rd? Or are your battens of constant cross section and bendy everywhere?
Constant cross section battens, untapered pultruded fiberglass battens for example, are too bendy in the leech area of the sail and often proper control of the leech is lost. Sometimes the leech can fall off, other times it can hook back to windward.
Good luck,
Bill