This can happen when the initial app sync is either syncing a lot of data (in which case waiting should resolve the issue) or it is failing to connect. Ensure your server config settings are correct on the Deploy Server screen. In particular, localhost will work as the server address for macOS and iOS apps, but not for Android. Your FMS console should show you the actual IP address.
The deployed app is just a blank screen
Updated Oct 28, 2020