Palczewski, A.D. JACoW Geneva, Switzerland Within Nb material batches which otherwise conform to specification, a distinct variability of susceptibility to pinning of residual magnetic field and thus enhanced RF losses and lower Q has been observed. Systematic investigation of the various lots of material procured for use in the fabrication of LCLS-II cavities is underway to distinguish the critical contributing material characteristic. The aim is clarification of a revision to the material specification in order to reliably obtain lower residual resistance in operational SRF cavities for any application.