The problem with anti-depressants and therapeutic response is that not every medication works for everyone in the same way. Medications that produce a great therapeutic response in one person may not do anything for another person -- and may even worsen the depression.
It can be a tricky balance finding the right combinations of medications to get the right response. Of course, that's true with many medications and especially if someone is taking meds for different conditions. A doctor's challenge is to make sure the medications are producing a good therapeutic response without interacting with anything else the patient is taking.