Perhaps you should try some spelling variations:
"improving voluntary performance risk." » "improving country performance risk.", "improving entry performance risk.", "improving budgetary performance risk.", "improving voluntary performance size.", "improving voluntary performance based.", "improving voluntary performance _."
"improving firm performance review." » "improving firm performance evidence.", "improving _ performance review.", "improving firms performance review.", "improving air performance review.", "improving birth performance review."

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving voluntary performance risk.") or ("improving firm performance review."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving voluntary performance risk.") OR ("improving firm performance review.")).
• Removing quotes may allow a broader search: (( improving voluntary performance risk. ) or ( improving firm performance review. )).
• Adding a wildcard symbol may retrieve word variants: (("improving voluntary performance risk.") or ("improving firm performance review."))*.