you should try a relaxer that has a higher strength so you can avoid underprossed hair (ex. maybe regular instead of mild). Or if you have a good relaxer, leave it in longer until its time. Usually the relaxer container has how long you should keep the relaxer in your hair based on your hair texture (fine, medium, coarse). I get a relaxer with Elasta QP and keep it in my hair for 15 minutes at most to avoid underprocessed hair. When I would wait like 5 mins (maybe because of burning), after washing the relaxer out, I had underprocessed hair.