This can happen when the queries in use is not marked as a selection. Follow the steps below to mark the query as a selection.
  1. Click Analysis > information library > Click on your query
  2. Click edit definition to open the edit ad-hoc query screen
  3. Click the set save options tab
  4. Mark create a selection and show this selection in the query designer
  5. Click save and close
Now go back to the information library and try to merge the two queries.