The electron transfer kinetics are fast enough to maintain the electrode surface concentrations of Reactant at the values required by the Nernst equation. But the heterogeneous electron transfer rate constant (ks) and the rate of change of potential - deopend on the scan rate and concentration. The process is said to be quasi-reversible in voltammetry. A quasi-reversible process is characterized by peak to peak separation DEp > 59.2/n mV, with the value increasing with increasing concentration or scan rate.

May be due to form protective layers such as metal oxides.

we also see this with regards stripping voltammetry and comment on our understanding in the attached paper where we looked at Pb deposition on boron doped diamond - in brief we relate it back to the morphology of the deposits on the surface  (size dependant redox potential) - others have also commented on this observation in the literature - in our paper we undertake a microscopic study and show how morphology of deposits really effects peak position - 

At higher concentrations, the peak current reachs a limiting value suggesting an electrode surface saturation and surface metal coverage on electrode surface. Charge transfer resistance ( Rct) aldo depends on concentration of metal ions.