When should i increase my dose of antidepressants?

Hey, I started taking 20 mgs of Celexa for depression roughly 6 weeks ago. The first 3 weeks I felt a minor effect, and week 4 was pretty nice, but lately it seems like I am feeling down once again. Not as bad as when I started taking them, but it feels like I'm heading that way again. Obviously I will consult my doctor regarding my dosage, but I wanted your opinions as well.