Evidence against the hypothesis [of chemical imbalance] comes from the efficacy of a newly developed antidepressant, Stablon (Tianeptine), which decreases levels of serotonin at synapses. Indeed, in different experiments, activation or blockage of certain serotonin receptors has improved or worsened depression symptoms in an unpredictable manner. A further challenge to the chemical imbalance hypothesis is that many depressed people are not helped by SSRIs.
https://www.scientificamerican.com/article/is-depression-just-bad-chemistry/
Know Thyself 