I'm not sure what could be causing the problem if it's happening with the mouse so we'll need a bit more information. Could you let us know which operating system you are using and the details of your computer (including whether it is a laptop, a slate type computer, a desktop computer etc.).
Could you also let us know if the problem persists after restarting the application and whether you see any problems with other applications not allowing drag (for example, can you drag a selection on your desktop).
That information should help us track down what's happening and find a solution.