Here are a few things I would have tried, that may fix this problem.
1. Gluten fri diet
It is possible that gluten intolerance or wheat allergy is triggering the condition. It is the common trigger for other similar skin problems like KP. Try the diet for 20 days if there is an improvement, you got your trigger.
2. Milk fri diet
It is possible that milk intolerance or milk allergy is triggering the condition. It is the common trigger for other similar skin problems like KP.
3. Blood Type Diet
It is possible that other foods in diet are triggering the condition.
It could be soy, eggs, tomatoes, strawberries, raspberries, nuts, peanuts, shrimps, Sugar added to foods, food additives , food preservatives ,