How to stop brown spots on white washing?

For 3 months now thw white wash has been ruined by brown spots (about 3mm diameter).
1. Spots do not wash out - bleaching stain remover is needed.
2. It occurs in my old washing machine and my new one.
3. It is not by cross contamination from dirty items.
4. My water supply is fairly hard but no recent change
5. Could it be related to the modern gel capsules?