have you used oxy clean and let it soak?

https://www.hometalk.com/42215787/q-how-to-clean-white-shirts-and-the-arm-pit-stains

This homemade oxiclean recipe should do the trick: https://www.craftylittlegnome.com/homemade-oxiclean-recipe/

Hello. This advice might be helpful.

Yellowing Arm Pit Stains -

Make a paste of 3% Hydrogen Peroxide and baking soda. Place onto the stain and let it sit for 10 minutes or more. Lightly scrub at the stain (a spoon or toothbrush works great). Shake off the paste and launder.

Squirt 3% Hydrogen Peroxide onto the blood stain and let sit for 10 or more minutes. Blot with a damp cloth and see if your stain is gone, if not repeat until it is. Once the stain is gone - launder as usual.

If everything in the load is white, add a cup of lemon juice to your load.