Downloads are often slower in-game due to how our API is implemented (to prevent downloads from affecting game performance) - this is something we are looking into optimizing in future updates to our SDK and plugins.
As such, sometimes the large file sizes of some of the mods available can take some time to download, as mods will download in the background only while the game is running, at a fixed transfer speed - please give plenty of time for these mods to appear while the game is running, or if you don't wish to wait, we suggest perhaps choosing some of the smaller mods that are available instead.