Perhaps you should try some spelling variations:
"improving maternal performance series." » "improving national performance series.", "improving thermal performance series.", "improving lanterns performance series.", "improving maternal performance metrics.", "improving maternal performance reviews.", "improving maternal performance review.", "improve maternal performance series.", "improving water performance series."
"improving mental performance series." » "improving senegal performance series.", "improving annual performance series.", "improving entry performance series.", "improving mental performance metrics.", "improving mental performance reviews.", "improving mental performance review.", "improving social performance series.", "improving rural performance series.", "improving fiscal performance series."

Try widening your search to All Fields.

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

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