no ear..help please!!

HI,

can't figure out what has changed for me.

I bake a 75% hydration sourdough dour(90%white, 10% rye: preheated  pizza staone for 1 hour, bake at 460 degrees, covered with steel bowl for 15, then  uncover for rest of bake).

My slahes are opening flat across the boule...no ear...... no rise up. Ideas please!!!!

thanks, Liz