It's expected that you can't see all boards in the timeline view. The timeline feature is still in beta, so I'd suggest providing feedback directly to Trello. When opening the timeline view in a board, you'll see an option to add feedback.
In the meantime, there are some alternatives:
1. Use Planyway or Team Gantt - I believe they both offer the option to see multiple boards in a timeline view. They will most likely be paid solutions.
2. Sync all your boards into a master board using Unito and use the timeline view in that board.
Hope that helps!
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