Problem: Windows will not let you span a window across multiple monitors. If you try, it will always "snap" the window to a single monitor.
While this can be handy in some instances, it actively prevents you from stretching a window across multiple monitors. There are lots of reasons why you would want to do this.
Solution: You may be using an AMD graphics card driver that uses the Catalyst Pro Control Center. If so, go to Catalyst Pro Control Center / Hydravision and check "Allow spanning across displays" in the Extended Display Window Control section.
Aside from that, I do not know. Check your graphics card drivers along with any desktop managers you might be using to see if there is a similar feature preventing window spanning.