I created a Checklist in My Library but it doesn't show up in my Case. Why not?

The most likely reason for this is that the Checklist was added after you created the Case in which you would like to use it.  You will not be able to access any Checklists that were created after a Case was initially set up.

But you can re-create the Checklist in a Case by selecting the Case Specific (Customize) option and entering the items.  Read more about creating and using Checklists.