'This Accessory May Not Be Supported' Issues

I keep getting the 'This Accessory May Not Be Supported' message when trying to charge my iPhone 7 Plus with auxiliary battery chargers. When one doesn't work, I'll plug in another make and it'll often work - for a little while. Then, swapping back to the original one that 'wasn't supported' often does the trick - but not for too long either!

I've tried the 'plug in and turn Airplane Mode on/off' method to no avail. Is there something else? I've thought it might be that the contacts on the phones charging receptacle needed cleaning, but then that doesn't make sense, because it charges fine at home.

Any thoughts or ideas? Thanks!