Stay-dry liners are usually made from a synthetic of some sort (polyester basically). This type of material is rather infamous for holding on to odors more than natural fabrics. I'd try just maybe putting them through an additional hot wash/rinse, either with no detergent if you suspect detergent build-up (synthetics are more prone to this as well) or with a smidge of detergent if you suspect they're just not getting as clean as the rest of the diapers.

HTH,
Karen.