One of the reasons this was missed is because of the way we test for Celiac. Since doctors measure blood (IgA, IgG)for gluten reactions only certain types of reactions can be seen. The reaction to the gluten in Corn doesn't show up the same way.
Celiac can come in a variety of forms
We are still learning so much about Celiac every day, however, we have a long way to go until everything is understood. Since some of those with celiac are not getting better, even with a traditional gluten free diet. It means that something in the diet is still causing issues. If this is you or someone you know then I strongly suggest that you cut corn and all things containing corn or "Maze" from your diet. This can be really hard since corn is in just about everything that doesn't contain gluten. By trying to live Gluten and corn free this will very much narrow down what you can eat. However, if it makes you feel better than it is worth it. If cutting corn is not helping then you might need to go back to see your doctor to find out what is going on. Also you might look into the possibility of lactose intolerance.