Perhaps you should try some spelling variations:
"improving thermal performance reviews." » "improving thermal performance review.", "improving thermal performance reports.", "improving thermal performance risks.", "improving external performance reviews.", "improving animal performance reviews.", "improving erm performance reviews."
"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 ("improving thermal performance reviews."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving mental performance series.") OR ("improving thermal performance reviews.")).
• Removing quotes may allow a broader search: (( improving mental performance series. ) or ( improving thermal performance reviews. )).
• Adding a wildcard symbol may retrieve word variants: (("improving mental performance series.") or ("improving thermal performance reviews."))*.