water rings on new

Can you help?  We just installed antique brown granite and was told it
was sealed.  However, within what seems like minutes, if a glass of
water is left on the counter, we have a water ring mark (a shade lighter
than the dark brown).  It's so frustrating after such an expense.  The
installer is no help. It seems that he did seal it because the water
beads on the surface when I test it. Why are the stains happening and
what can we do?  If you can help I would be so grateful. Nancy