Preview Apps not being able be installed / not appearing

I am trying to do playtesting using release channels.

It is working fine for most of my users, but some cannot install the game. When they go to their "preview apps" section of their account (which is nearly impossible to find), it shows nothing, even though they have accepted it.

Some even see the ability to switch from LIVE to "Preview" in the release version, but once in their headset or app, still cannot install the game.

Please advise. Or let me know where they can go to get support.