These problems are often  related to authentication issue or configuration issue.

So please follow these steps :

• Connect to the CRM Server, then run deployment manager, select your organization and click browse from the right panel and then navigate to the Dependent Picklist link.
• Define mapping between « case type » & « priority »:

• Create XML web resource
• Click on get script button and copy the xml
• Create a new xml web resource

• Edit case form
• Edit case form, and click on form properties
• Add pgs_SDK.DependentOptionSet.js script
• Then add new function on the form load

• Then click on the form click on “case type” field and add onchange event

• Publish customization
• Test

Open a case (an existing one or a new one)