The problem: You are looking at the Timeline view and don't see a certain activity.
Possibility 1: First, make sure you have clicked on the results framework (or technical area) that your activity is mapped to. Find this on the left-hand side of the Timeline view. Second, make sure that you have selected "Any mechanism type" (or that activity's mechanism type) and "Any status" (or that activity's status). In the example below, the activities shown on the timeline are mapped to the framework called "Ultimate outcome".
If your activity still does not appear, make sure that the activity is assigned to a results framework part (or technical area) by assigning indicators to that activity. If your activity does not report on any indicators, then you can assign it to a technical area but uncheck any related indicators.
Possibility 2: Additionally, activities must have start and end dates listed on their Overview page in order to show up on the Timeline view.
If the activity has start and end dates listed, make sure that the time period selection window (the blue rectangle at the top of the Timeline view) includes the dates when your activity is active. Then, both the activity and any Calendar or Checklist items created for that activity will appear on the Timeline View.