Why a metal rod shows no charge when rubbed on other object?

Short Answer: Because metal rods are conductors. This means that while they easily accept a charge, they also easily discharge. Thus, any charge that is applied to it will immediately discharge, resulting in zero potential.

Long Answer: Search for 'tiboelectric series', 'electrostatics', and 'properties of conductors and insulators'. An in-depth answer would require a collegiate-level introduction to Physics and Calculus.