yellow stains

I'm probably in the wrong forum but hopefully someone can still help me out here.
How do I get these ugly yellow stains under the armpits out of my white T-Shirts??? I'm from Germany and have never had that problem at home. Our whites are being cooked for 1 1/2 hours and everything comes out bright white. Now here I have the biggest problems with my whites altogether but the biggest nightmare are those underarm stains. Any ideas would be more than appreciated...