Chewy Rice Balls in Pork-Cabbage Soup (Tang Yuan)

I must have done something wrong here.  I know I took some leeway when I substituted neck bones for the pork butt and simmered everything longer to obtain a gloriously cartilaginous broth.  That move I feel comfortable with.  I did have to keep adding water because the amount the recipe called for wasn't enough to submerge all of the other ingredients at any time.

What confounded me was the dumpling part.  I made the dough as directed: white rice flour plus warm water, mixed and rolled into balls just like the other rice-flour dumplings I've made from this book (onde onde, kube mutli).  The recipe said to drop them into boiling water, boil until they're all floating (about two minutes) and then cook another two minutes.

Well.

After not two minutes, but about an hour of energetic boiling, these leaden dumplings still sat obstinately on the bottom of the pot.  Not stuck - just sitting there.  They had long since turned the cooking-water into a pale starchy soup but refused to float.  I tested them every twenty minutes or so and observed the uncooked center spheres becoming smaller and smaller, but they never decided to cook all the way through.  

End result?  The soup was a little too salty but otherwise delicious, and the dumplings were very, very chewy.  Given the opportunity to use pork neck bones again, I think I'd just stick to plain rice.