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Well if your maintaining the same keyword positions but seeing less traffic it could be less search this year, changes to your meta descriptions which have messed your CTR up, ppc ads for those terms more competitive therefore better offers in the ads (discount codes etc). Can you say one of the keywords you are ranking for which has not moved but you have seen a decrease in traffic for? If an affiliate gets crawled more often then you, you might find when you add new products or content which is inside of your datafeed (networks pull this every 24 hours) and the affiliate does say a cron job on your feed every couple hours they could have your content picked up before you.

Appreciate your response... many thanks