On Sat, 13 Sep 2014 15:29:33 +0200, nestork
wrote:

If it's a new flapper, run your finger over the seat it closes against,
and if you feel anything except a smooth surface for the flapper to
close against, that's the problem.

Could be just some slime build up and the flapper is not closing
entirely. Easy to wipe clean with a rag. Possible the flapper just
needs a minor tweak from side to side, to ensure it seals correctly.