My food allergy was missed by my gastro and immunologist. It took my elimination diet (Specific Carb Diet) and a dermatologist to figure out that I was allergic to tree nuts.
Originally, I was having problems getting better on a gluten-free diet. My gastro sent me to an immunologist who did allergy testing via prick tests and blood tests, but I had weird results when I didn't react to any of the pricks, including the histamine "control." It has something to do with antibody deficiency... so I still don't know everything that I'm allergic to (if any). But those pricks only tell you what you're allergic to, not intolerant or sensitive to.
Through the diet I was able to see that when I added nuts into my diet I broke out into hives, eczema, rhinitus, throat swelling, and other "allergy" symptoms. I went to the dermatologist for meds to control the eczema, and he helped me tie together the symptoms to the food trigger. When I added corn, I developed gastro issues, brain fog, lethargy, neuro issues, among others. For me - nuts is an allergy, and corn is an intolerance.
Good luck!