When I have issues with the underarms of my workout shirts coming out still discolored from sweat+deodorant, I like to take a small amount of laundry detergent = just enough to dampen the areas needing extra help. Then I put it in the washer, add my detergent for the load, with a presoak setting so the clothes all soak in the water and detergent for 15 minutes before the wash cycle actually starting.