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In my case Google seems to have gotten better at penalising certain areas on my site and leaving others untouched. My guess is that they are tweaking the duplicate content filter as I take product information direct from the merchants' datafeeds.
MSN and Yahoo don't seem to have a problem with it. Just this so called new and improved Google.
I don't know if its a simple as that.
As an example, on 22 sep our site disappeared for a few days and then came back as normal.
This continued until early march and then it disappeared again, not ranking at all well on any DC.
Also, site: command returned all sorts of bizzare results, huge page numbers etc and home page not listed as the first in the list, dmoz description appearing now and again.
On the 16th of may this all seemed to change, 90% of the DC's were showing our pre-march positions, with very little change amoungst the other sites listed for "our" search terms.
Then as we have all seen this changed back on the 18th.
The site: command on DC's giving favourable results for our site returns the rightish number of pages, with our home page listed first correct description etc.
The other DC's - the not so favvourable ones show the junk when site: command is run.
So I can only assume that Google is doing a lot of work in the background to sort this out. It would be nice if, while they were doing this they showed our favourable results :).
Hopefully they will come back when they have resolved their issues.
I do think it's unexpected issues that they are dealing with, basic infra-structure changes should not have this kind of effect.
Unless we are all being led up the garden path and really there has been an algo change as well, which Matt's recent post regarding "who you link to and receive links from" would suggest. An update in all but name.
Again, I don't think its a question of just a filter as I'm sure many more sites in the area we monitor would also be affected.
What that means is anyones guess?
I see this same effect for my ailing site too. If I am tripping some kind of filter, I can't figure out what it is. But I am caught in some kind of Google hell and have been for quite a long time.
I think at this point, all I can do is continue to make changes and hope I change whatever it is which the Google God doesn't like about my site.
My nice pre dec 27th rankings are still on these centers:
I have noticed that these DCs have now given in and become like all of the others. Are you seeing this?
Still had some hope that they may have been testing something on these DCs prior to a full rollout. It's looking increasingly unlikely now :(