you may need to replace the parts that sit between the bowl and the tank. There are just two bolts to unscrew after draining the tank. Make sure the tank itself is clean (debris will prevent a seal on the flapper) and that the center tube is high enough that water is not draining through that when the tank is full. The typical rebuild should last between 6 months and a year if you have very hard water, or maybe several years if your water is less alkaline and mostly chlorine free.