My fairly new IPhone 6S Plus (just bought it in early July) has begun doing something odd recently. I'll turn my phone off at night, and when I go to turn it on in the morning, nothing will happen. The only way I can get it to actually turn on is to plug in the power adapter then hold down the power button, and it'll fire right off. Once it's powered on, I can then unplug the adapter and it continues to run perfectly fine, and the battery shows 94% remaining, so it's not like the battery was dead. Any idea what could be happening here and how to get it to power up correctly without needing to plug it in? Thanks.