This usually occurs because the platform checks whether there are any personlisations in the message that it would need to pull a value for form the list. If a column for a specific personalisation value isn't available then the platform won't let you use that list with that message.
For example, say a list contains the following columns:
... and the message contains the following personalisation values:
You can see that the FavouriteColour personalisation value isn't available in the list, so the platform won't let you schedule a campaign using this combination, and you won't see the list.