Discoloured collar and cuffs on school dress

[Astrid]

All the white parts on DD1's check school dresses are getting discoloured. Not sure what it is, probably just a combo of sweat and dirt.

I have tried soaking and scrubbing with sard soap, but they have not made a dint in it.

Not sure if it is due to having to use a sensitive powder and soaker, and/or being on tank water. DH does wonder if the washing powders are made to interact with the chemicals in town water, hence why I struggle with white laundry.

Any ideas?