You could try more sensitive substrates. Do you have info about the peroxidase (assuming you're using HRP, Reinheitszahl)? How do you coat your plate? Are you immobilizing the capture antibody directly or with a third AB catching the capture AB? Any azide acidentally present?

I use a secondary antibody HRP. I use the TMB slow kinetics from sigma for detection. I coat the plate using a human monoclonal antibody directed against my protein, the capture antibody is directly immobilized on plastic. There is no azide in my ELISA.

homo Arginine Elisa kit.