Positive reinforcement is what worked for both of mine. You said you give treats when she goes outside. I would stick to that. Really a lot of praise and petting and treats when she does it right. Hang in there she will get it! 
__________________
We give dogs time we can spare, space we can spare and love we can spare. And in return, dogs give us their all. It's the best deal man has ever made. -M. Acklam |