perhaps they were referring to that the image was not exposed properly(either overexposed or underexposed) may be too contrasty or lacking of contrast,but obviously something to do with exposure so if you think you can not correct it by re-editing it then it'd be better to retake.
good luck
edit:also you know they are not so keen on harsh shadows and unwanted reflections
very true. i have to agree stokfoto!
StockXpert sells for me, so if they say improve lighting/exposure,
i take that as qualified advice not to shoot in that lighting situation.
saves me the trouble , instead of submitting shots for nothing.
