they always DO!!!
but the fact is due the predominant effect of the LARGE metallic plate, we cannot observe the effect of that force and the 2 charges experience force wrt the metallic plate!
two point charges are kept in space.
they exert force on each other the magnitude and direction given by coulomb's law.
now, if an infinitely large metallic plate of considerable thickness is placed between the charges, do they continue to exert electrostatic force on each other?
they always DO!!!
but the fact is due the predominant effect of the LARGE metallic plate, we cannot observe the effect of that force and the 2 charges experience force wrt the metallic plate!