I drew the geometry on a flat plane and then tried to extrude to the desired thickness from each side. One side extrudes well but when I go to the reverse side, the closed face has disappeared. If I redraw lines I can close the face and extrude but that seems very cumbersome.
If I were modeling this I would give the arm the thickness along the center before adding the details to the sides. Since it is symmetrical I would model one half of it and then copy and clip it to make the other half.
It looks like you started by drawing a 2D version on a helper face and then began pushpulling the various parts? That’s a good way to keep the initial drawing all on one plane, but you need to delete or isolate the rest of the helper face before doing pushpull because when you pushpull an interior shape on a face, SketchUp assumes you mean to cut an opening and deletes the starting shape. You can avoid this by pressing ctrl (alt on Mac) before doing each pushpull.
As @DaveR notes, it would also be faster to model half and then flip a copy to create the other symmetric half.
Thanks for the tip on pressing ctrl when extruding to avoid opening up the face on the opposite side. It helps a lot with my labor intensive technique of developing geometry on opposite faces.
I will be following up on Dave’s suggestion to work on one face and then copy the geometry to the opposite face. Sounds like that should cut my work in half.
Point taken. I was just reviewing the fundamentals course, and I see in the section on the push/pull tool that they identify the ctrl modifier key with push/pull to leave a copy of the surface. It seems obvious now but I know I missed that before.