Pushing Apps Alpha/Beta Channels

I am trying to push my app to 4 headsets through the alpha channel. The users have accepted the invite that was sent to them. Some of the devices have received the app while some have not. I have unsubscribed them and subscribed them again, restarted their headsets, ensured all the headsets have the latest updates and ensured the wifi was working correctly.

This has been ongoing for a number of apps over the past few weeks.
Could you let me know how to resolve this issue or is there another way to push the app to the headsets?