I recently used 4 of the Cobalt Extra Barding Slot unlocks in various ways, and they were all consumed but failed to unlock the extra barding slots on my Aerakyn's Ashengale Wings mount. CS returned the items to me, and I am posting this here as the CS GM requested I do.
Mounts have been funky for a long time. To change / add bardings after zoning around, I've always had to dismount, logout and log back in, then resummon the mount to be able to make changes.