Just clearing something up.
Your rhomboids are located behind your trapezius muscles right so they will never be visible.
Your traps run from around the base of your neck to your middle back creating that diamoind shape and the fibres run in different directions mainly horizontaly.
So really the traps is the muscle that you use to shrug your shoulders up and down but they also bring the shoulder blades together towards the spine and the rhomboids only assist in this.
When doing BB rows for instance you should feel it heavily in your traps since you have to retract your shoulder blades towards each other and the traps is the main muscle that does this?
I think when im doing BB rows i get pissed off because i feel it more in my upper traps rather then nearer down my middle back but this is normal isnt it lol