objects are missing glTF

The curtain is there, but only visible from the back
Vectorworks: My Exported Object is Invisible or Only Visible from Certain Views - Electronic Theatre Controls Inc

with a right click on your model in the hierarchy you can expand it to see all the parts of the model. i can see two times "Ridå", i assume that means curtain.

Yes, that is them. Some how A3D can't read the info that Windows can. it is not that it's lost in export, it is lost in import. I will do some testing with different textures and so to see what will happen.

A bigger problem for me is the floor... Did a workaround where I made the A3D floor smaller and only fit my stage floor. But I would like it to work without it.  

Yes, that is them. Some how A3D can't read the info that Windows can. it is not that it's lost in export, it is lost in import. I will do some testing with different textures and so to see what will happen.

A bigger problem for me is the floor... Did a workaround where I made the A3D floor smaller and only fit my stage floor. But I would like it to work without it.  

In this export, everything is marked as double-sided.
Most likely very little of it actually needs to be - usually you don't look up through the floor, and when you look in from above the ceiling you usually want to be able to see the inside!

Augment3d does not yet support double-sided materials as they interfere with lighting (Windows Paint3D doens't do lighting), so only the 'front' side will be shown and lit.

Most 3D modelling programs that make it clear which is the front and which is the back and have a way to flip it around.

Ok, so I will have to flip them around? The floor in my drawing is a box, Guess I have to find a way around that.

Cleaning up some old posts, have a look at this support article for some additional info and tools to help correct the model: Imported 3D Scenic Models Appear Transparent/Invisible