The bran in whole wheat usually sucks up a ton of water. If, say, you do 65 percent hydration normally, bump it up 5 percent. That usually works for me. If you go by volume, just add a tablespoon of extra water at a time and mix. Keep doing that until it feels more soft but holds its shake. Your starter should be fine, though. I use a 60 percent hydration stiff starter that starts out as hard dough. It's more active in yeast and tastes better to me. Although, you have to alter hydration calculations and use more water.