[SalesForce] Not able to add field for related list in page layout

It seems like there's a standard related list called 'Cases' on Opportunity. This picks up all the sibling cases (via the Account parent). That is, it picks up all Cases related to the same Account that the Opportunity is related to. We'd added the standard Cases related list instead of the custom one we created.

We had some old case records which had a different account on case than the opportunity which is why they didn't show up in the related list.

Also, we now have some logic in place which updates the Account on Case from the one in Opportunity whenever the Opportunity field is changed - this ensured both Accounts were the same and we couldn't replicate the issue!

We have now renamed our custom related list to something more appropriate to our business :)

1) I am not too sure WHY those field are not available. Looks like this functionality is not available yet according to this.

2) You can use <apex:relatedList> tag to display related list of records on a VF page