Can't See All Tables in Entities Pane

Why is it that only SOME of the tables in my database show up in the Entities pane?

There are a total of six tables in my database, yet only three of them are available to me in the Entities pane. I have looked at permissions, etc, and I am using the SA account. It seems like I should see either ALL or NONE of the tables, but that is not the case.

What am I missing?

Only tables that you included in the Data Source View when you built the Report Model will show in the list of entities. Did you include all six tables? If not, or if you have added tables since you built the model, you'll need to build the model again.