I had same issue. It was difficult to get the heart rate up on a rower. The key is to put the resistance down! (You mentioned putting the resistance to highest

I had same issue. It was difficult to get the heart rate up on a rower. The key is to put the resistance down! (You mentioned putting the resistance to highest setting). At a higher resistance setting, my back muscles would fatigue before my heart rate went up. By reducing resistance, stroke rate will go up, less muscles fatigue and more cardio.