For anyone following up,
Alvesj wrote into the help desk about this, and we found the issue was stemming from the device's individual settings, rather than Axure RP. Some of the iPads were set to prevent videos from auto playing, which was why some iPads worked and some didn't. The solution going forward was to first update each device's iOS version so that they're all running the latest or close to the latest, which allows each iPad to have the same configurable settings. The setting in question can be found by opening up "Settings > iTunes & App Store > Video Autoplay" on iOS devices.
Hopefully this helps!