Yeah. This is a thing that can be frustrating.

It basically boils down to not reserving an instance for an individual, but instead trying to "tetris together" parties from what's available to send out asap. Think of it like filling rides at a theme park.

So, assuming you are a DPS, you will need 7 other members, including 2 healers and a tank who:

A) Specifically want to go to your dungeon or
B) Doing roulette with the required iLVL and/or
C) Don't essentially get "poached" by other groups.

As I understand it, you may be entitled to the next group that comes along, and say you have everything but 1 healer... then someone queues for an easier roulette and all they need is a tank. They're ready to go, and the tank that's currently in your group signed up for anything (roulette), so they're gone and you're stuck needing a tank AND a healer now. This can go back and forth until everything clicks in place.