I don't know exactly what "unspecified" refers to, but if you mean ER-staining outside the nucleus, you should generally think to:

staining procedures (antibody incubation time and/or concentration);

endogenous reaction (peroxidase, biotin);

cross-reaction (molecular similarity between antigens - as personally observed with PR which cross-reacts with intestinal mucin or squamous epithelium cytoplasm).

In the first two issues, you should manage to optimize time, Ab dilution and peroxidase/biotin blocks. In the third case you can check the H&E slide to compare morphology details and exclude cross-reactions from ER evaluation.

Hope you find this useful.