Perhaps you should try some spelling variations:
"improving plant performance contract." » "improving plant performance contracts.", "improving port performance contract.", "improving court performance contract.", "improving land performance contract.", "improving place performance contract.", "improved plant performance contract."
"improving tax performance series." » "improving tax performance studies.", "improving tax performance size.", "improving box performance series.", "improving cta performance series.", "improving dam performance series.", "improving trade performance series.", "improving data performance series.", "improving care 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 tax performance series.") or ("improving plant performance contract."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving tax performance series.") OR ("improving plant performance contract.")).
• Removing quotes may allow a broader search: (( improving tax performance series. ) or ( improving plant performance contract. )).
• Adding a wildcard symbol may retrieve word variants: (("improving tax performance series.") or ("improving plant performance contract."))*.