Some people have found that only a certain number of tasks can be put into the timeline view.
Fortunately Heather has posted a workaround"
To fix this issue you’ll have to re-create your timeline view, to do this:
1.Take a screenshot of your timeline view so you can remember how you had the tasks arranged.
2.Remove the timeline view from the current view by going to the View tab, Split View group, and un-checking timeline.
3.Go to File – Organizer and delete Timeline from the right side (the box that says the name of your project)
4.Go to the View tab, Split View group, and check Timeline.
Your timeline view should be working again and you will now be able to add tasks to it.
We have fixed this issue in the Project 2010 June Cumulative Update. You can install that from here http://support.microsoft.com/kb/2075992/ This update won’t fix any corrupted timeline’s but it will prevent future ones from getting corrupted.
I'm posting this here so I can find it again!