Navigation through entities in the report model is determined in the design stage. If you are navigating and entities and corresponding fields disappear there is a problem with the design. However, you can access all of the fields by using the "search" feature. If you haven't used it before there is a little magnifying glass to click on. It's pretty handy. I don't believe there aren't any books on how to use the report model and it will not be supported by Microsoft in the future.
Click here to see Technet article about SQL Server 2012.
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