DNA transfection Mammalian cells

I would like to regulate the expression of a gene and in order to do that, I have purchased specific siRNA. After optimizing my transfection protocol and using electroporation I have achieved a 60-70% reduction of the gene of interest. However, I cannot observe a significant reduction of mRNA expression but only a reduction of protein. What might be the problem? Could the problem be in my cell treatment method?

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