There's a theory about
"Brain Plasticity" and Antidepressants that sheds light on why antidepressants take so long to work. The theory is growing by leaps and bounds. If simply making the neurotransmitter available was the mode of action, people would experience relief from depression practically over night. This isn't the original article that I found on the topic, but it's close.
www.learning-knowledge.com/plasticity.html