How do I stop toilet leak?

I changed my old toilet and added a new Jacuzzi elongated toilet. The old toilet did not leak. To my surprise, there is no toilet flange on the floor. The 4" pvc drain is about 3/8 inch below the tiled floor surface. The two bolts are cemented into the concrete floor. The home is built on a slab. I used an oversized wax ring and had to shim for rocking and the toilet did not leak for 3 months. Now it leaks a little bit. I thought about stacking two regular wax rings but am unsure on what to do. Any ideas???