Hi There!
So, which one did you replaced? The rubber one or the circle pad part?
If you did replaced the circle pad it self, make sure that you calibrated it correctly on the 3DS it self. If this didn't fixed it, then return it and get yourself a new one.
If it's the rubber, make sure that it's inserted correctly. Also, make sure that the plastic rings are placed back as it was before! Otherwise it could cause the issue you are having.
Otherwise, replace the plastic rings.
Please let us know how it goes!