My wife's S6 battery life has become atrocious, and it is not fast charging anymore. The phone is 10 months old.
I have checked the settings, and fast charging is active.
I would just reccomend ordering yourself a new Micro USB cable as Samsung's do brake after a few months. Ive gone through a load, buy a good quality one, you can get USB that go inside both sides so no worrying about putting it in one way.