Oh that's odd! ~~I've never had a recipe that didn't call for adding sugar to activate it. I guess it was relying on the tiny amount of sugar already in the milk?~~ If you're sure it's not rising enough, adding a tablespoon (or less) of sugar is definitely worth a try!
Edit: okay wow I'm sorry, it's been a while since I made that, so I went and double checked my recipe and I do not remember doing it the way it calls for! Apparently I mixed the active dry yeast in with the dry ingredients (including both sugar and flour) and then mixed a warm milk and butter mixture into that. Definitely unusual and not my normal breadmaking process!
Still, I think the amount of sugar in milk can naturally vary, so if your recipe has you proofing the yeast (unlike mine, I guess!) adding a little extra might help?
Good luck!
I'm gonna put the recipe I used into the spoiler here, in case you want something to compare against.
Chocolate Babka Dough:
This makes 2 loaves.
Chocolate Filling:
Streusel:
Egg Wash:
Directions
Prepare the dough: Warm milk and melt butter in a glass or ceramic bowl in the microwave for 30 seconds, or in a saucepan on the stovetop. Combine bread flour, all-purpose flour, yeast, and sugar. Add water, milk-butter mixture, egg, and salt to the dry ingredients and mix well.
Use the dough hook in a stand mixer on low speed or knead the dough by hand until the dough is smooth and elastic, about 15 minutes. If the dough is too wet, add more bread flour, about 2 tablespoons at a time (up to 8 tablespoons) to make a workable dough; too much flour can make the dough dry. Cover the dough with a damp cloth and let rise until double in size, about 1 to 1 ½ hours.
Make chocolate filling and streusel while the dough is rising.
For the filling: Stir together finely chopped chocolate, cinnamon, and sugar. Cut in chilled butter with a fork.
Make the streusel: Combine confectioners' sugar and all-purpose flour; cut in chilled butter until the mixture resembles coarse crumbs.
When the dough has doubled, punch the dough down and cut it into two equal pieces. Loosely shape each piece into a ball. Cover the dough with a damp cloth and let it rest for 10 minutes.
Divide dough in half; roll each out to a 12-inch square on a lightly floured surface. Spoon 1/2 of the filling onto each dough square and spread to within 1/2 inch of the edges. Roll up each square tightly as for a jelly roll. Pinch ends and seams closed to seal. With your palms, roll logs back and forth until uniformly round.
With a sharp knife or dough scraper, cut 1 log in half lengthwise to form 2 striped strands. Working quickly, twist strands loosely together with cut sides facing out, making 2 or 3 wide, horizontal twists. Fit into one of the prepared pans, patting back any loose filling and tucking ends under, if needed. It might look like a mess now, but it comes out beautifully!
Repeat with the second piece of dough.
Cover with damp kitchen towels and let rise until doubled in size, about 30 minutes.
Preheat the oven to 350 degrees F (175 degrees C).
Make the egg wash: Combine egg and water in a small dish and whisk to combine.
Brush the loaves with egg wash. Sprinkle streusel on top. Bake the loaves, rotating the baking sheets to promote even browning, until the bread is a deep golden brown, about 25 minutes.