Why can't I see all the available timelines when I create an iteration plan?
One answer
As you didn't mention how you created the plan, such as from Eclipse client or webui, so I can only guess the possible reason.
When you create a plan which owned by a team area, you could only see the timeline which is associated to the team area.
In addition, when you create an iteration but do not select "A release is scheduled for this iteration", you could not create a plan for the iteration.