Short answer
It's usually the adapter, the cable, heat, or iOS deliberately pausing at 80% to protect the battery.
Isolate one variable at a time and you'll usually find the cause in minutes.
Common causes
- A low-wattage adapter — old 5W cubes charge very slowly.
- A damaged or uncertified cable.
- Heat: iOS slows charging when the phone is warm.
- Optimised Charging or an 80% charge limit (Settings › Battery › Charging).
- Lint in the USB-C port.
Fix it step by step
- Try a different wall outlet and a known-good USB-C power adapter of at least 20W.
- Swap the cable.
- Check Settings › Battery › Charging for an 80% limit.
- Let the phone cool down before charging.
- Look in the port with a torch. Don't dig with metal; a technician can clean it safely.
- Restart and try wireless (MagSafe or Qi2) charging to see whether the port is the issue.