Because I am a new member I was not allowed to continue posting to a thread so I am spinning off to hopefully help @XLR8R .
Background: after successfully pairing the chromecast with the harmony hub some of the control buttons were not working. Here was my solution.
I was having a similar problem with buttons not working. Here is what I did to fix.
I removed the chromecast from the device list and re-did the setup it completely from the harmony app.
When I paired the Bluetooth I noticed that after the pairing was complete it disconnected from the “harmony keyboard”. So although it was paired it was not connected. On the chromecast click on the harmony keyboard and select connect.
I hope that solves it for you