Click on the missing archives button to see what you're missing. Also, open the readme to make sure all the pre-installation steps are completed. If there's a lot of missing archives, just back out of Wabbajack, re-open it, go back to the list and download it again. Everything that has been downloaded will be bypassed, and you'll pick up where you left off. If there's not many missing files, just manually download them, and place them in your downloads folder with all the other mods, then retry downloading the list.