It appears for some people there is a problem where the devices won't charge if the Lightning cable is plugged in while the phone is asleep, which seems to be caused by a software bug.
As reported by The Verge, it means the handset needs to be unlocked - or have the screen lit up - to begin charging.
Already, many users have been looking into workarounds to battle the issue, including upgrading to the iOS 12.1 beta.
It's said that the patch eliminates the problem, so it shouldn't affect users once Apple releases it to everyone.
Other suggestions include going into Settings, then FaceID and Passcode, before scrolling down and turning on USB Accessories under the 'Allow access when locked' option.
This latest issue follows uuser complaining about poor signal and WiFi reception in the US compared to older models, with posts on MacRumors, Reddit and Apple's own suport forums.