Removing Stains from Hospital Pads?

Once again I am asking for your expertise as any previous solutions have worked so well. I use 'hospital pads' around the house for my dogs to lay on when they are chewing a bone or treat of any kind. I wash them regularly, however, once in a while the stains get quite bad. Is there something that I may soak them in, in order to get them cleaner? Thank you, have a nice day and stay safe.