It is actually well known in psychiatric circles that AD's, SSRIs in particular are not that effective. This was to be expected as the serotonin system in the CNS is like a dimmer switch and exhibits minimal control over overall brain functioning; it is also a very small set of neurons compared to many other systems.
It is drug companies that try to make SSRIs seem so effective. I would like to add that even though SSRIs over all are not so hot, they help alot of people. I have seen lives changed by these meds.