The "serotonin hypothesis" is largely unfounded. We don't actually know why SSRIs are effective for some people. It's likely more to do with increased neuroplasticity than a shortage of serotonin in the brain.

This is a good study to start with: https://pubmed.ncbi.nlm.nih.gov/35854107/

> The main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations.

Prescribing antidepressants is a shot in the dark. Doctors can't know for sure if you'll respond to them, especially because we don't even really understand how they work.