Overview

You discovered that some leads were removed from your Plays in Playbooks, and when you are trying to add them back you cannot do it. 

Information

To add leads from Salesforce to Playbooks, You have to make sure you own the record (lead) first. If you don't, you just need to change the lead to you ownership to be able to add the lead to Playbooks.

If you have to change the owner of multiple leads, make sure you check the Change the Owner or Status of Multiple Leads at Once article in the Salesforce help center.