I think the softest kolaches often have potato in the dough—here are the ingredients for Weikels in La Grange:
Bleached flour, water, soybean oil, egg yolks, sugar, potato flour (potato flakes, mono diglycerides, sapp, sodium metsbisulfite, citric acid), margarine, citric acid, artificial flavor, beta carotene, vitamin a, yeast, salt, dry milk.
I found a similar recipe, although I haven’t had a chance to try it yet: http://midwesternmoms.com/2011/07/kolache-recipe/
I also bookmarked this a while back: http://www.texasmonthly.com/story/we-gotcha-kolache
You might also be getting too much flour in your dough if they are all coming out hard. You might try a different method of measuring the flour to see if you get a better result.