So, how do I get more masks?

I just came back to WoW recently and my character has gotten strong enough to do a full Vision of Org/SW and I got the Mask of the Long Night last week but was too intimidated to use it. I looked up on Wowhead how to get the other masks and realized I needed to use the first one, so I unlocked the bottom of the Titan Panel Research and thought I would give it a go to use the Long Night mask to get the Mask of the Burned Bridge, and Mask of the Daredevil. I cleared every zone and killed Thrall, but at the end, none of the chests had another mask in it, then when I used my last vessel for the week, I didnt even get a new Mask of the Long Night. What do?