Customer can't view parts on output page

One of our customers cannot see a part for a certain product on our part list output page, but I can. What is causing that?

Each part has a setting to allow a customer to view it or not. Go to the product in question (Settings -> Catalog -> Products), then click on the parts tab and find the part you'd like the customer to be able to view. Click on the arrow next to that part and make sure the option to 'allow customers to see this part' is checked.