Do you know why I can't thicken this knitted surface?
I think there is a problem with the tangency of the highlighted boundary surface.
You are correct.
I would suggest fully defining your sketches, applying additional guide curves, loft sections or change your surfacing method.
Those corner 3D sketch guides are part of your problem. If you want to create a bulge at the base of the pouch you'll need to let that centerline tangency extend out further. Does your pouch have a vacuum seal around the entire border?
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