The most common cause of dumplings becoming too dense is because of too much gluten or you have put too much dumplings in the pot to cook at the same time.
When you put too many dumplings into one pot at a time then they can become overcrowded which can cause the dumplings to get too dense.
So try cooking less dumplings at one time and avoid overcrowding the pot with the dumplings.
If your dumplings still are too dense even when they are not crowded then you should try a different flour such as a low gluten flour such as cake flour or use all-purpose flour but substitute a couple of tablespoons with cornstarch.
When I've used cake flour to make dumplings they started turning out just fine but with regular flour they would be to dense.
My Grandma always used cake flour when making her chicken and dumplings and she made great chicken and dumplings.