Hmmm... not sure. It can be finicky at times. Did you still have Kies installed? Try uninstalling the program. It could possibly be interfering with Odin.
Also, try opening the program in Administrator mode (Right click -> Run as Administrator) prior to plugging in the phone (which should already be in download mode) to the PC.
And try a different USB port if you've done all of the above and it still doesn't work.