Maybe there is an issue with your composer file. You could try:
composer update always regenerates composer.lock and installs the lastest versions of available packages based on composer.json
composer dump-autoload won’t download a thing. It just regenerates the list of all classes that need to be included in the project (autoload_classmap.php). Ideal for when you have a new class inside your project. Ideally, you execute composer dump-autoload -o , for a faster load of your webpages. The only reason it is not default, is because it takes a bit longer to generate (but is only slightly noticeable)