Could you try changing the litter box to a shorter hieght? some cats have difficulty or pain getting in/out of the box. How often do you change the litter box? sometimes the urine gets into the plastic and it really smells. Make sure you have cleaned the area where she has gone throughly with a cleaner that has enzymes in in.
You could also try this litter:
Tabitha April 10, 1995 - August 23, 2013
Bomber April 10, 1995 - July 12, 2010
Winston Nov 15, 1999 - September 15, 2011
Sophie Aug 30, 2011
"UNTIL ONE HAS LOVED AN ANIMAL, PART OF THEIR SOUL REMAINS UNAWAKENED"
He is your friend, your partner, your defender, your dog. You are his life, his love, his leader. He will be yours, faithful and true, to the last beat of his heart. You owe it to him to be worthy of such devotion.