Have you ever checked placing a report with object prompt in report service document? if so, then the object prompt won’t appear in document.

Tips:

1. Place all the attributes (which present in object prompt) in to the report
2. Add as a data set into report service document.
3. Drag and drop the report(grid) into the document.
4. Save and close the document.
5. Open the report, remove all the attributes and place the object prompt into the same place (column or row) where attributes are placed.
6. Save and close the report.
7. Now, execute the document.

No wonder, it will ask you to select the attributes from the list.