why the current in cyclic voltammetry is decreased when the electrode was modified?
It is not true for all the electrochemical reactions. Depends upon active surface area of electrode.
It depends on both the material that you deposit on the electrode surface and the redox reaction that you investigate. Various phenomena can account for this matter but the expermental conditions must be specified in advance. For instance, if the modifer is an insulator, it may block the electrode surface and hamper the electron transfer process. Besides, it may get involved in the reaction and change the underlying mechanism of the electrode process. So, as I said, more information is required for justifying the current-potential behavior.
The closest related with material that you deposit on the electrode, please used the new material and repit this experiment.
Sometimes deposition may increase charge transfer resistance of your modified electrode materials (you can check with EIS). Catalytic activity of materials also may loss. So, one need to be careful selecting modifier materials.
If you are doing polymerization then check current after 5,10,15.. cycles. See for how many cycles current is maximum. Choose that number of cycles for polymerization.
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