There is a known issue which can sometimes cause minions to be unactivatable if you interact with them too quickly after playing them (before the enter play animation is done). This is most obvious with charge and rush minions but can happen with others too (if you somehow can interact with the minion on the turn it is played). Some have reported having laggy connections, there could be a link there.
The main way to avoid that is to wait until the animation is completely done before attacking. If you encounter the issue, you can disconnect and reconnect quickly (on Windows, I use Alt+F4 to kill Hearthstone; in Mac OS, maybe command+Q will work), this will refresh the client and make the minion work normally again. Hopefully you catch it early enough in your turn that it reconnects before the rope times out.
Another forum member posted earlier a link indicating that the issue may be fixed soon, fingers crossed!