Not all Roadmap items show in the timeline view, but they do show in the grid view in the New User Experience (UX).
Example: Roadmap starts in 2020 and has a duration of 1 year. The project that doesn't show on the timeline starts in 2019 and ends in 2021. The project shows on the timeline view if the Roadmap duration includes either the start date or the end date of the project.
STEPS TO REPRODUCE:
Actual Results: The project is missing from the timeline view, but does show in the grid and board views
This is working as designed in Clarity 15.9.1. Below as per the criteria listed in New User Experience: Top-Down Planning with Roadmaps from the section: Display Scope for Roadmap Items in the Timeline
Release: 15.9.1 and lower
This behavior was modified to allow for this in 15.9.2
Update the Roadmap Start Period and Duration to cover the start and finish dates of the roadmap items in the Roadmap.
Example: If a roadmap item's start date is in 2015 and the finish date is in 2019, change the Start Period to 2015 and the Duration to 5 years.