Some dogs have genetic or neurological based issues. We do our best to be realistic with our clients and let them know if we feel there is a greater issue at hand that training cannot solve. We have wonderful vets on recommendation to assist when these sorts of issues present. We always take an honest approach where we are realistic with attainable behaviors and goals so that our clients are never wasting their money.
Furthermore, many dogs are simply management cases. For instance, a dog with genetic dog aggression may never like the company of other dogs. However, they should be able to behave on leash and display non-reactive behavior on walks.