In making my starter for your recipe with 13g ripe starter with milk and flour - it is very stiff. Can't imagine that its going to grow any???
Hi, which recipe are you following? Something doesn't sound right. There can be variations in flour, some more absorbent than others. A stiff starter can also ferment. Not all starters are soft paste or batters or resemble dough.
If it feels too stiff, stiffer than any dough, work in more liquids, either milk or water so it can ferment. A look at the recipe would be helpful along with information about the flour.