If you have multiple domains that target different countries that have the same language (e.G. mydoamin.nl & mydomain.be), and therefore the same keywords, do you think that it is possible that if one of the domains ranks for a keyword it is kind of blocking the other domain from being shown in the SERPs? I've experienced that at some point but I was never sure if my own domains were competing with each other or there were other reasons for it. Of course the correct html_lang tag was given for both. Did you experience something similar?