Perhaps you should try some spelling variations:
"sources research series" » "resources research series", "services research series", "sciences research series", "sources research services", "sources research theories", "sources research studies"
"source research series" » "force research series", "source search series", "resources research series", "source research services", "source research theories", "source research studies"

Try widening your search to All Fields.

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

• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"source research series\") OR (\"sources research series\"))".
• Removing quotes may allow a broader search: (( source research series ) OR ( sources research series )).
• Adding a wildcard symbol may retrieve word variants: (("source research series") OR ("sources research series"))*.