thin shells

Hallo,

can anyone tell me what my mistake is...I´ve been trying to create a thin Shell construction from an Experiment just like Heinz Isler once did but my Shell is concentrate in the middle...and that´s not what the Shell looks like..and I don´t know how to repair it.. Also I was wondering how to pick more than 4 anchor Points.. because the geometry only recognize the end Points and I Need more than one Point at the corner to avoid singularity.
Thank you guys!!