You did not assign physics. Go to 3D > Fabric Manager . You need to assign a .fbr (fabric property) file. If you don't have one, set Bending value to 50 and Weight to 150 g/sm . Default zero values cause rigid simulation.
A comprehensive guide for learning Optitex involves mastering three main areas: the Pattern Design System (PDS) for creating patterns, the for virtual draping, and the Marker Module for production layout. Core Learning Resources Official Documentation Optitex Help Center optitex tutorial pdf
The Pattern Design Software (PDS) is your canvas. The PDF will teach you: Optitex Tutorial PDF — Structured Content Outline Fix