I know that a particular food makes me sick, but it didn’t show up as an allergy with the allergy test. Why?

If you know that you have a symptom-causing food that does not show up on an IgE allergy test, it could be a food sensitivity. Specifically for food, an IgE mediated food allergy and food sensitivity are not the same thing. However, food sensitivities currently cannot be measured on any blood testing method. So you won’t find the answer to your sensitivity question in an IgE allergy test, and definitely not in an IgG test (that is an unproven test). To find any food sensitivities that you suspect, you should consider an elimination challenge to them.