Is the top of the flange even (or close to even) with the finished floor? If it to low – then use two wax rings. One regular wax ring on the bottom and one (or more) with the plastic horn insert on top. ) I have seen leaks like you describe if the glued flange is not really glued in all the way. Take a look at that – if your floor and flange is flush. Sometimes you need to shim the toilet if the floor is uneven or the flange is to high.