Please follow the following steps if a case is not visible or searchable in the portals:
1. In order for a case to be visible and searchable in partner portal, there is a global org setting named 'New Cases Visible in portal' (checkbox) which must be checked.
2. The setting can be found under 'Setup | Customize | Self-Service | Self-Service Portal| Settings'. On this page, in the 'Enable Login to Self-Service Portal' section, navigate to 'Self-Service Setup' button.
3. Here, in 'Self-Service portal default settings' section, 'New Cases Visible in Portal' checkbox must be checked for the cases to appear in the partner portal.
4. Now create a test case record and make a portal user the case owner and test by logging in as that user in the portal whether the case is visible.
5. Once the checkbox in step 3 is checked and logged in as the portal user; the user can indeed view the case in 'My Cases' list view.
6. This process only updates visibility for new cases created after the 'New Cases Visible in Portal' checkbox is checked. For existing cases, update the 'IsVisibleInSelfService' flag to 'true' with data loader for the cases to be searchable on portal.