Hi, Indy. Let me offer a couple of things to try. First, make sure that the pull chain is letting the flapper go all the way closed. Then go to a plumbing supply house and buy the best flapper you can get. I have had the same problem with cheap ones from a home center. The rubber is so hard that it doesn't seal well. I forget the brand name but the red rubber ones work well for me. There are some slightly different ones, depending on the attachment, so be sure that you have the right one. Then check it for flash from the casting process, particularly in the pivot holes. Finally, shut the water off, flush and remove the old flapper. Put some scouring powder on a rag and clean the hole at the bottom of the tank thoroughly. Could be some scale there causing it not to seal. Then install the new flapper, turn on the water and check the action. Recheck the chain and be sure that it is not catching on something. Hopefully, one of these things will do it for you.