You have to check if you painted your glass on the front face or on the back face of your window. Then you must also check what is the material on the front face.
If you click on the face of window and look at the entity info what materials do you see there? The left square is front face, the right square is the back face.
Then:
If your glass is in the back face, try assigning it to the front face too. If the front face is unpainted (double color material) you might have assigned a material to the component/group where the glass face is inside and you should remove that material assigned to the group/component by painting it with the default material. Do that for every group component in the model that is nesting the window.From now on you should assign materials directly to faces unless you know what you're doing.