Had a look at the page you linked, it all looks okay for me, the debug info shows that your AWS keys are valid and you are looking for similar products to ASIN: B006O6FGTE.
Of course for me it is showing the UK items, so I recreated amazon links on my site using the appropriate ASIN and spoofed it to show the .es links and products and it worked okay.
I did browse a few other pages and I see that it is not working for the product "B006O6FHNY".
I went to http://www.amazon.co.uk and added that item to my basket and Amazon also does not show any recommendations, so it looks to me like it is not showing any recommendations as Amazon has no recommendations to give!
It works better if you can give it more data, e.g. have more than one product on the page. If you don't want to do that you could have a hidden shortcode that contains the best selling product in that category. e.g. just before the recommended product section add:
If you are doing large numbers of Amazon requests they might get throttled back by Amazon, which would make things intermittent.
Turning on the Amazon Link Cache will help with that, although the 'Similarities' lookup always has to be done live for each visitor.
Hope this helps!