iOS 9 has increased the security around Over the Air (OTA) installations of apps. Your iOS9 participants must now indicate in their device settings that they “trust” your Apple Enterprise developer certificate before they can install a build.
This additional step is only required at the initial installation of your app. Subsequent app installations will be marked as trusted.
Please see below the steps your participants will be required by iOS9 to install the app.
- Go to Setting -> General –> Profiles
- Tap on the dev. certificate in the “ENTERPRISE APP” section
- Press “Trust [Developer]”
- Tap “Trust”
- Proceed to install the build