iMessage not working between two specific devices
First, check the destination address. On your iPad: open Messages, then open the message thread that's not working. Tap the "Contact" button (top right), then the "info" button (i). The destination address will be highlighted blue.

Next, make sure the destination address is configured on the receiving device. On the iPod: Settings &rarr; Messages &rarr; Send &amp; Receive. If the destination address is missing, add it here.

And while you're at it, check the FaceTime settings, too: it works in a similar way.