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

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