Based on your pics, it looks like the washers on the input jacks are in the wrong order. The washer with the metal ground tab should be sandwiched between the plastic washer and the nut. It looks like you have this reversed, with the ground tab washer sandwiched between the chassis and the plastic washer. If so, the ground tabs on both Red and Black inputs may be isolated from the shell of the input jacks or only making intermittent contact.
cheers,
Derek