Perhaps you should try some spelling variations:
"improving maternal performance series." » "improving materials performance series.", "improving external performance series.", "improving national performance series.", "improve maternal performance series.", "improving water performance series.", "improving maternal performance studies.", "improving maternal performance size."
"improving mental performance risk." » "improving general performance risk.", "improving senegal performance risk.", "improving annual performance risk.", "improving fiscal performance risk.", "improving gender performance risk.", "improving data performance risk.", "improving mental performance 2019."

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