I have added a couple of vertical links on my schedule but they do not appear.
On the View tab, be sure that Task Links is checked.
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Related topics:
Three ways to add vertical links
Number of Vertical Links
Vertical connectors in the legend
Vertical connectors on a multi-page schedule
Delete vertical links from a symbol (Milestones Help)
Vertical link properties (Milestones Help)
Hide or show vertical links (Milestones Help)