I had a similar problem with a Cintiq 21UX that I bought used, and I thought I had been totally ripped off. It worked great the first day I got it and never worked again after that. I tried EVERY suggestion on every forum there is...uninstalling and reinstalling different drivers, restarting the Wacom "service" and all of that. So here's my recommendation if you've done all that and still no luck.

Check your graphics card.

When you unplug the Cintiq's DVI monitor cable from your computer you'll see 18 round pins and one wide, flat pin. In my case, that wide flat pin had gotten pushed inward because the female end (on my graphics card) was just slightly too narrow for the pin to insert.

After many days of cussing out the guy who sold me this thing, and losing my mind because nothing was working, I discovered this little issue. So I used needle-nosed pliers to pull that wide flat pin back out a bit, and swapped out my graphics card with older one, and it fixed the problem.

I feel like I may be the only person on earth who has had this particular problem, but if this post helps even ONE other person, it will make my day.