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In Webmaster Tools for example we actually show more links coming in for the singular, let's use a generic word like "Gift", but it just keeps falling as opposed to "Gifts" which is still very high.
Could it be
A. the purchasing of new links for "Gift" , (but we did it for both)
B. Over/under staurated metas and keywords.
C. Adding footers on the homepage to go to pages of content under the respective keywords.
D. Some other penalty.
I see some sites that rank and don't even have the keywords very prominent on the homepage or any metas on their sites, but do use the footers. Others have the keyword all over the homepage, but no footers etc. Some use H1's some don't. That's why it is easy to say it could be all the above, but what is it most likely. Tough to figure out what to do. What have you noticed to make one keyword 3rd and the singular version 40 when both used to be 3rd for example.
Thanks in advance. J
1. Single word results are really different - whether singular or plural. However, I'm assuming you are talking about a multi-word phrase here.
2. Google has been working to understand user intention - and there is often a difference between user intention on a singular or a plural term. Wish I could be more specific, but it seems to me that this particular venture of Google'sis rather term-specific.
Neither of those areas would necessarily men that you've got a penalty of any kind. If these changes are mission critical, I'd suggest studying the urls that are still ranking well for your target terms like a madman.