A domain for each language or a single domain, which is better for SEO?
There are two types of method for multilingual websites.
First is using sub-domain for each language (the same as using separate domains).
As you may know, each sub-domains ranking separately and has no affect on your root domain. Google identify each sub-domain as an independent website. Normally you may experience situation as like as you mentioned. I have already multilingual website and suggest next type:
Second is using sub-directory for each languages, it is more popular in most cases because google identify all languages of your website as a single website.
In this case each sub-directory try to get ranking higher not only for themselves but also for other directories and also your root domain. As a result, I suggest you to use subdirectory and redirect all your domains to new addresses.