Somewhere on line, a blogger studied various thread brands under a microscope . . .and the surprise was, not all 50wt thread is equal. Most brands, even at 50 or 60wt, while light weight, weren't dense enough, but rather bulkier than others. The one that was super dense and the one I get the best results with, is Aurifil 50 wt.
But, I also use, at the recommendation of Edyta Sitar, Schmetz Microtex 70 needles (and change needles after every full bobbin is emptied on my big machine, and every other bobbin on my normal size bobbins)
The needle hole is smaller . . .and on batiks especially, when the hole often stays put, this makes a difference.