Perhaps you should try some spelling variations:
"improve maternal performance series." » "improve materials performance series.", "improve external performance series.", "improve national performance series.", "improve maternal performance studies.", "improve maternal performance size.", "improving maternal performance series.", "improve water performance series."
"improving mental performance series." » "improving mental performance studies.", "improving mental performance size.", "improving general performance series.", "improving senegal performance series.", "improving annual performance series.", "improving maternal performance series.", "improving fiscal performance series.", "improving gender 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 ("improve maternal performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving mental performance series.") OR ("improve maternal performance series.")).
• Removing quotes may allow a broader search: (( improving mental performance series. ) or ( improve maternal performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving mental performance series.") or ("improve maternal performance series."))*.