In Salesforce, Picklist fields can be either controlling or dependent fields. A controlling field controls the available values in one or more corresponding dependent fields. A dependent field displays values based on the value selected in its corresponding controlling field.
For example, I've a custom object called as
Expense__c. In that I've two picklist called as
Controlling Picklist and
Controlling Picklist controls the values which are suppose to be show in dependent one.
Now if I try to include dependent picklist on page layout, it won't show any values since it's controlling parent is not available to select. So whenever you're working with these kinds of picklist, make sure you're including it's controlling picklist as well.
To do that, Go to Setup-> Search "Object" in quick find -> Select targer object -> find page layout -> edit -> Find controlling picklist -> drag it in page section -> save it.
For more information on Field depencies and Modifying Page Layouts