Take a patient with anxiety. The manifestation and cause of anxiety can differ wildly from one person to the next. Some patients respond well to pharmacology and only a few visits to alleviate their symptoms, others need a mix of pharmacology and cognitive therapy, and some do well with only therapy. The hardest part is figuring out which is best for the patient. Some cases are harder to treat than others.
Expectations have to be constantly revised. We're not lie detectors and sometimes people expect us to be. Therapy is a two way street and you can't help everyone.