It’s likely one of three things:
1. The charger (the square-shaped thing that plugs into the wall outlet)
2. The lightning cable (the cable that plugs into the phone)
3. The battery itself. Apple’s iPhone batteries are designed to retain 80% of their original capacity for 500 full charge cycles. If you’re still using the original battery that came with your 6 Plus that could be the issue.
What to do...Try charging your phone with a different charger and cable. If that doesn’t work take your phone into an Apple Store, an Apple authorized repair center, or contact
Apple Support.