Perhaps you should try some spelling variations:
"central america research nejm series." » "central american research nejm series.", "central americans research nejm series.", "central africa research nejm series.", "central america research new series.", "central america research need series.", "central america research needs series."
"central cgiar research film series." » "central asia research film series.", "central asian research film series.", "central city research film series.", "central cgiar research firm series.", "central cgiar research field series.", "central cgiar research filip series.", "central cgiar research tilsm series.", "central cgiar research forum series.", "central cgiar research film studies."

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (("central cgiar research film series.") OR ("central america research nejm series."))~.
• Removing quotes may allow a broader search: (( central cgiar research film series. ) or ( central america research nejm series. ))~.
• Adding a wildcard symbol may retrieve word variants: (("central cgiar research film series.") or ("central america research nejm series."))*.