A common mistake with the dumbbell row is retracting the shoulder blade and holding it there throughout the lift. The elbow often ends up behind the body. This can lead to shoulder pain.
What you want to do is "unglue" the shoulder blades with the dumbbell row. They should move around the ribcage.
Pull the shoulder blades down and together, but don't let the elbow go past the midline of the body.
At the bottom of the movement, get a bit of a stretch, even allowing the dumbbell to come to a full stop. This will get your shoulder blade moving correctly.
The motion of the row shouldn't be straight up and down but more of an arc from forward to back. You'll hit your back harder and your shoulders will be healthier in the long run.