You can probably relate when I say that, in the beginning, I had a very hard time growing my back muscles. Most people (I did it to...) tend to lock their shoulder blades into a fixed position (pulled back is often the choice), and then row or pull down without letting them move out of place.
In my opinion, and unless you have any condition/injury that doesn't allow you to do it, whether you’re doing a row or a pull-up/pulldown variation, let your shoulder blades move freely!! This means letting the weight pull your shoulders up (in pulldowns) or down/forward (in rows), and then pulling the weight by squeezing your shoulder blades together.
Why should you do it? Well, dynamic contractions (where the muscle lengthens and then shortens) are better for muscle growth than isometric ones (where the muscle maintains a certain length throughout the movement). Before explaining any further, just let me give you some anatomical wisdom that will help you understand things better. The two main muscles of your back are your trapezius (or traps) and your latissimus dorsi (or lats):
The traps can be divided into 3 portions: upper, middle and lower. The middle and lower portions are the ones we'll focus now and they both originate on the vertebrae (T1-T12) and attach to the shoulder blades, so they're responsible for scapular retraction and depression (pulling the shoulder blades back and down, respectively).
The lats have several origin points (the bottom of the shoulder blades and T7-T12 vertebrae are two of them) and insert in the humerus, your upper arm bone. Some of its fibers also contribute to scapular retraction and depression.
When you're doing rows, if you fix your shoulder blades in place, your middle and lower traps will stay at a certain length to keep them in that position. However, if you let your shoulder blades move away from each other, your traps will lengthen and then actively contract to pull them together, and that will be a much better stimulus for growth.
The same goes for any kind of pull-up/pulldown variation. You should let the weight pull your shoulders up so that your lats can fully lengthen. Then, pull your shoulder blades back and down, actively contracting the lower fibers of your lats to move the weight.
Bottom line, let your shoulder blades move freely through their full ROM. If you usually fix your shoulder blades in place, start slow. First, just let them move a bit, and then progressively move them further and further away, always controlling the weight.