Decreasing magnitude of peaks in cyclic voltammogram?
Perhaps your observation is an indication of irreversible reaction conditions.  More specifically, a tight deposit on the electrodes hampering the advancement of the electrochemical reactions. This happens sometimes with some less soluble organic dyes used organic electronics, too.  Take a look if the electrodes are still shiny, maybe they need to be cleaned again.  If this is the case, try out a different solvent/electrolyte system.

One aiding companion to trouble shoot is this.