why I cant see all the fields related to a salesforce case?

This is likely due to permission settings. The Salesforce connection in Clay is scoped to the user who created the OAuth connection. You can only see fields that this OAuth user has access to. If there are specific Case fields you need to access, ensure the OAuth user connecting Clay to Salesforce has permission to view those fields.

Sources: FAQ: What permissions and scope do I need for the Salesforce enrichment? Salesforce integration overview | Documentation | Clay University