Purity would affect superconduction. Consider grains of superconducting material embedded in a matrix of regular conductive metal.
One of the theories for the behavior of this material explains that the copper atoms preferentially form a structure that does not superconduct. The structure that does superconduct is tricky to achieve due to the energy levels. (Paraphrased summary)
It's likely that a poorly prepared sample will have discontinuous regions of both superconducting and non-superconducting material. If that is the case, you won't observe superconduction.
It may be that the non-superconducting material does superconduct at low temperatures, which would mask the purity problem.
The truth is that quite literally nobody fully understands what's actually happening here. That's kind of the point of all this experimentation
One of the theories for the behavior of this material explains that the copper atoms preferentially form a structure that does not superconduct. The structure that does superconduct is tricky to achieve due to the energy levels. (Paraphrased summary)
It's likely that a poorly prepared sample will have discontinuous regions of both superconducting and non-superconducting material. If that is the case, you won't observe superconduction.
It may be that the non-superconducting material does superconduct at low temperatures, which would mask the purity problem.
The truth is that quite literally nobody fully understands what's actually happening here. That's kind of the point of all this experimentation