If iTunes is not responding to your commands
If you are able to connect and view the media library from your remote device but the playback controls do not respond, check to see if iTunes is displaying a dialog on your computer. When iTunes displays a dialog box it blocks applications like Signal from communicating with it. Closing the dialog should allow Signal to recover automatically.
If no dialog is displayed and you are using Windows you may have encountered a rare iTunes bug where it does not properly register the programming interface that allows applications like Signal to control it. To resolve this exit out of iTunes, Signal, and any other programs that communicate with iTunes, then re-install iTunes. This will typically resolve the issue and Signal will then be able to control iTunes.