JDownloader by default doesn’t start active downloads once it is launched.
In order to enable this :
1. Start JDownloader and select the Settings tab.
2. Under “Downloads & Connections” section, enable the check mark next to “Start Downloads on Startup”.
This will now automatically begin active downloads whenever JDownloader is launched.
Also, to increase the number of parallel downloads that can be done from different file hosts, at the bottom right corner of JDownloader window, increase the number next to “Max Dls” to preferred choice.