The hub to disc mating surface needs to be absolutely free of any rust or other contamination when replacing the discs. My guess is that the this is the source of the problem.
It could be warped discs if they are of poor quality or have been abused. I think that the only real answer is to check for run-out on each disc and either replace the discs on the axle concerned or, as a first pass, remove each discs and clean the mating surfaces until they are spotless.
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