However, three weeks is a long-time, it's possible your mood is dropping and worth discussing this with your doctor.
(Fluoxetine) has a very long half
, possibly up to eight weeks, and so does take much longer than most SSRI's to 'kick in'. Also, it's thought any individual anti-depressant will only produce symptom relief in about two-thirds of depressed individuals who take it. Again, perhaps speak with your prescriber.