Thank you all for your very useful comments! Yesterday we went sailing again, and we've checked the tension in the rig (there is enough tension we believe) we did try to pull the downhaul with tension on the sheet. Indeed this gives a little more (we didn't know to do it this way) This way we flattened the sail out while on shore.

Then we went sailing (but with 14 knts) the boat ran very nice, and no problems at all. So we believe you're right! We need to put more power on the main sheet with more wind. This kind of sail apparenently does not accept to sail it more "relax" (the old Inter sail was more capable of that)

Of course we have to try it with more wind, and when that day comes, I'll come back on it.