The iOS device (iPhone/iPod/iPad) is not docked properly.
Remove your iOS device (iPhone/iPod/iPad) from the connector and check for any obstruction on the connectors in the dock and on your iOS device (iPhone/iPod/iPad). Then reseat it in the dock.
Your iOS device (iPhone/iPod/iPad) is not playing.
Press play button on your iOS device (iPhone/iPod/iPad) to play it manually.
The iOS device (iPhone/iPod/iPad) is locked up.
Please make sure your iOS device (iPhone/iPod/iPad) is working properly before docking it to the unit. Please refer to your manual for details.