If you’re using the original charger that came with the iPhone 6s, it will be a 5W adapter only capable of churning out 1 amp (+-0.5). Try using the 20W USB-C charger or any of the iPad chargers and see if it’ll take up to 1.6. If you’re using a genuine cable and adapter capable of over 2 amp output and still facing this issue, it may be a problem with the charging port or battery. Replacing the battery first would likely be the easiest and cheapest way and is more likely to be the issue in this case, however if not you’ll have a new battery as well as a new charger port by the end ;)