Please check your cables. We have found that cables must be Apple or at least Apple-certified for iOS devices.
Also, please remember that the nature of a test harness is NOT exactly the same as experienced by a ''normal'' user -- the devices are always plugged in and powered on, meaning that the cables are at least delivering a ''trickle charge'' at all times. It is plausible to have a cable go ''bad'' from time to time and especially an intermittent (sometimes good/sometimes bad) cable can cause much frustration when attempting to run tests against various devices.
We recommend labeling all cables with the ''source'' (i.e. Apple, Apple certified, Samsung), and date obtained and to have extra ''good'' cables on hand for swapping out with suspected ''bad'' cables. This approach may also be necessary with USB hubs used as part of the test harness.
Much troubleshooting time can be saved by adopting this ''known good'' equipment on-hand approach. When hardware is suspected at issue, you can swap out (at least temporarily) known-good equipment or cables for troubleshooting.
I hope this helps.