I have looked in tools preferences en IMDB but dee no reference to the IMDB ratings.
There are no preference settings affecting
IMDb Rating. It can change over time as more users vote, so it's updated every time the plugin is run. There is no way to download just one field. The plugin always downloads the entire web page, then updates fields according to the overwrite and other configuration settings.
So, you'll need to look a little further for an answer. If no rating appears after running the plugin, check the website to see if there actually is a rating. New and/or obscure entries do not show ratings until a minimum number of votes have been recorded. If there is a rating, but the plugin will not update it, give us the URL so we can investigate. Finally, it's possible the plugin was not working for a time (because of changes to the format of the website), but works now. If that's the case, use
Search to find movies with
IMDb Rating = 0, select all of them, and run the plugin to update them.