Assuming that the fast cable charging option has not been disabled in the battery settings, the only failures to fast-charge I've ever experienced with my previous S6 Edge, and my current S7 Edge, have been due to USB cable selection and in one case a faulty (brand new) Samsung OEM fast charging-adapter that would only standard-charge the devices.
Samsung replaced the defective charger, and in each of the other cases replacing the cable has resolved the issue.
Don't know if you have 3rd-party USB cables, but if so be sure to choose only those that offer at least 24AWG (that is to say, a value smaller than or equal to 24AWG) conductors for their power-lines in the cable. My anecdotal experience has been that 28AWG implementations do not provide satisfactory operation with respect to fast-charging.