On the face of it that makes no sense. You can forget the Volume controls, they will work regardless of the remote selected as Slingplayer hijacks them for it's own volume. But the fact that the Power and Channel buttons are working means the correct remote is loaded. And normally a remote either doesn't work or works 100%.
But there have been a large number of posts about problems with the Comcast X1 so it is worth doing a Forum Search to see if anyone else has had the problem and solved it.