The middle-back area can be difficult to zero-in on, and establish a strong mind-muscle connection with, especially with more "traditional" back exercises. One of many things that I've learned from Dante Trudel, is that sometimes you have to "get weird with it". If you're doing the same exercises over and over again, and aren't seeing much growth in certain muscles, it's probably time to try something new. This could mean putting your body into mechanical positions that you haven't previously; the key is finding something that work for your unique genetic structure. The exercises below are two rowing variations that I've found very effective for myself and the majority of my clients, when it comes to developing the mid-back.
Prone Rack Rows
The body position and pulling angle of this variation, offer a good way to effectively target the mid-back area. I prefer performing these slow and controlled, "feeling" the muscles stretch and contract on every rep. Set at incline bench to about a 45 degree angle with the top end heading into the rack. Keep the bar firmly against the rack throughout the entire range of motion (pull up and back slightly). Focus on driving your elbows up high and wide while retracting the scapulae, as opposed to simply pulling with your arms. Imagine that someone has the tip of their finger placed directly in the center of your mid-back, and the goal is to squeeze it when you're in the contracted position. If you have a partner you could actually have them point a finger there.
View this exercise here:
Prone Mid-Back Cable Rows
These put you in a great position to effectively hit the mid-back as well. Set an incline bench back a few feet from dual cable tower, facing away from it. Grab the handles with a pronated grip and lie face down on the bench. Bring your elbows back high and wide while retracting the scapulae. Flex hard for a second once fully contracted, and let everything open with a big stretch when you return to the staring position. I'd recommend using the same tip of trying to squeeze a finger that's pointed directly in the center of your mid-back on each rep, regardless of one being there or not. This isn't a movement that I’d try to move heavy weight with, as you could easily lose the mind-muscle connection, along with the ability to work the target muscles effectively. Take your time, be conscious of where the exercise is being felt, and don't throw away reps.