Some of the reason why some channels of your organisation is not visible in the app are:
No content in your channel : Only channel with content is visible in the app. Try to add some content in the channel and you will see the channel in the app after restarting the app.
You are not a part of the channel: Only users who are part of the channel can see the channel in the app. If you are an organisation admin, you can access the channel in the backend but you need to be a part of the channel to access it in the app. Add yourself to the channel and you will see the channel in the app after restarting the app.
Please feel free to write to us at firstname.lastname@example.org if the problem is not resolved