It may seem like your queued downloads never start, and while this isn't true, it may take some time. The issue is that there is a background task that is responsible for checking queued downloads has to check hundreds of thousands or even millions of downloads that are queued, and this can take some time.
The background task is also very smart. We have live readings of the capacity and usage of all of the servers, and the background task will avoid adding downloads in times of low capacity or high usage, to make space for other downloads that are more urgent.
So what if you want to start your queued download instantly? You can, simply click on the download on the dashboard and click on the "Force Start" button. This will do exactly that, force the download of the files without waiting for a background process. This will only work if your account has enough slots, and the download passes all of the checks!