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Hi Evelien, Does organic traffic appear to be attributable to any particular country or countries? This may sound strange, but I wonder if a competitor pulled out of the market. It appears just about every competitor you have in google.be got a pretty nice organic increase around that time, which has continued. Kruidvat seems to have the lion's share now. Last I knew, Luxembourg was something of a tax shelter. With the recent changes in VAT, I wonder if a significant competitor or competitors found it difficult to continue operations. But that's just a 'shot in the dark' on my part.

All we can do here is guess, but my guess is that you had a temporary "honeymoon boost" when you first launched the site.  Google will often rank a new page highly, see how users interact with it and then drop it down in the rankings after a few weeks. I would imagine that that's a pretty competitive term, so to rank #7 for it would take a lot of authority!

Where are you getting the data from for the search volume difference? Secondly, what are you using to look at your rankings for both terms? Manual searches do not always give you the whole picture of what is ranking or not ranking. Before you restructure the whole site, set up a few test pages that focus on the non-plural version of your keyword. Monitor the ranking closely. If you see a large influx in traffic, then slowly implement change across your site, although I would not place all your eggs in one basket. Use some of the keyword idea tools available to look at broad match keywords related to photobook(s), and build out your content to cover additional areas. (if you have not already)