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Give it more time in a warmer spot rather than baking a dense dough; the rise time is a guide, not a rule, and a cool kitchen easily doubles it. Check that the milk was only lukewarm, since milk hot enough to feel warm on the wrist is fine but hotter milk kills the yeast. If the dough shows no movement at all after another hour, the yeast was likely dead or expired and it is worth starting over.

Yes - apricot, plum or cherry jam all work, since the jam is treated the same way regardless of flavour. Keep the cornstarch-and-water step no matter which jam you choose: it thickens the filling so it stays put instead of running out of the rolls in the oven. If your jam has large fruit pieces, chop or blend it first so it spreads evenly over the rectangle.

Store them in an airtight container at room temperature; the butter and egg in the dough keep them soft for a couple of days. Refrigerating dries yeast dough out, so avoid the fridge. They freeze well once fully cooled - warm them briefly in the oven and dust with fresh powdered sugar afterwards, since the first dusting dissolves.

Yes, the jam filling can be cooked with the cornstarch and chilled a day ahead - it needs to cool anyway before you spread it. You can also shape the rolls, place them on the lined tray and refrigerate them overnight, then let them come back to room temperature and puff up before brushing with the egg yolk mixture and baking. Bake the rolls themselves fresh, as the contrast between the soft crumb and the golden top is best on day one.

Most often the jam was still warm when it was spread, which makes it thin and runny; let it cool completely first. Leaving the 3-4 finger widths at the top free from jam matters too - that strip is what the cut strips fold over and seal with, and jam there stops it from sticking. Finally, roll from the bottom up snugly but without stretching the dough, and press the seam gently so the roll holds its shape.