I think that , most of us have or had refusals because of poor background removal.
An easy way to check it out is to use equalize option in Photoshop. After you removed your background and turned it to white make a copy of your layer and click
Image >Adjustments > Equalize.
If you see dots or stains on white partition of the image then undo equalize option and clean these areas with a white brush.
Thank you very much for reading