When upgrading from Joomla 1.5.X to 1.6 i ran through an issue with jUpgrade. It seem like it was always getting stuck in the Downloading process. After restarting the process different times with different variables, i still could not get past the Downloading step. So i figure i check the Parameters settings, there i was able to disable the Download process. After changing the setting i restarted the upgrade process and it work correctly.
If your Mootools Upgrade module is enable and jUpgrade keeps getting stuck, then try the following:
Go to your Components and click on jUpgrade. Click on the Parameters button. Looks like this:
Disable the Downloading step by selecting YES in the Skip Download setting.
Now retry the upgrade and at the end you should see this screen:
If you still can't get pass the download step, you might new to download the upgrade file yourself and upload it in the temp folder located in your Joomla directory.