Perhaps you should try some spelling variations:
"improving firm performance issues." » "improving firm performance measures.", "improving firm performance based.", "improving firm performance size.", "improving _ performance issues.", "improving firms performance issues.", "improving air performance issues.", "improving birth performance issues."
"improving human performance evidence." » "improving loan performance evidence.", "improving thermal performance evidence.", "improving company performance evidence.", "improving urban performance evidence.", "improving rural performance evidence."

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

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