Use food coloring in the tank and see if the colored water enters the bowl; if it does,,,that means the new flapper you installed does not match the brand name of toilet you have or the surface to where the flapper must match has an imperfection. Highly unlikely but I would venture to think that the flapper is not the right one.
Look at back of tank or on the flat surface behind toilet lid on the bowl to determine brand name and if possible, take old existing flapper to plumbing supply house to correctly match to manufacturer.