First, do these powerbanks support charging by Lightning, USB, or MagSafe/Qi2 wireless charging? I'm guessing they would be the Lightning-type unless you tell us otherwise.
Regardless, and based on what you've described, it does sound like the issue is likely with the iPhone rather than the power banks, especially since you've tried multiple power banks and different charging cables with the same result, while the power banks continue to charge other phones normally.
So why could this be happening?
- (Most likely) The charging port may have debris or lint inside it. Even if the phone charges from a wall charger, a power bank may not make the same quality connection. I suggest carefully inspecting the Lightning port with a flashlight and gently cleaning out any lint using a wooden or plastic toothpick or a soft anti-static brush. Avoid using anything metal. Note: If cleaning the port doesn't help, this may indicate wear or damage to the Lightning connector itself.
- The power bank may not be completing the charging "handshake." Some power banks automatically shut off if they don't detect enough power draw or if there is a communication issue.
- A software issue. If you haven't already, try restarting your iPhone. Also make sure you're running the latest version of iOS.
