I've been having a few problems with the Scale Automator action too (used it for the first time yesterday). I used it to create a droplet application that would copy the original, prompt me to choose the scaling percentage, then auto-rename the file by appending the date to it and place the scaled renamed, image copy on the desktop.
I found that if the original image file was in the target location (the Desktop) it wouldn't work at all (which makes sense, I guess). When I moved the droplet to my /Applications folder it stopped working completely until I restarted and now it works fine. So have you tried logging out and back in or restarting? Maybe that'll help?