There are typically three reasons why a member will not automatically receive an introduction when joining a hub that currently has Introductions running. Some hubs have members who initially opted out of Introductions. This setting can be altered by the member in their profile or on the hub home page.
Members will also not automatically receive an introduction if they join a hub most of the way through a cycle. This is to ensure that each member gets the maximum amount of time with each introduction.
There may also be no introductions available when a member first joins. They will receive an introduction as soon as one becomes available!