With Apple’s release of Security Update 2016-001 El Capitan and Security Update 2016-005 Yosemite (more info here; update ASAP, if you haven’t already), our Dock became stuck on one monitor of our dual monitor Mac setups running El Capitan.
Normally, the Dock will move from screen to screen simply by resting the mouse pointer at the bottom of the desired display for a few seconds. No such luck after the update.
Fortunately, it’s easy to quickly “unstick” your Dock by simply restarting it.
You don’t need to enter a command in the Terminal since you can quit/force-quit background processes using the Activity Monitor utility. Just click the CPU tab, select the Dock process (you can click Process Name to sort processes in alphabetical order to find it easily) and click the Quit button (the “X” icon) in the upper left of the Activity Monitor window. You will then be asked if you want to do a normal Quit or Force Quit. A normal “Quit” should do just fine.
Our stuck Dock restarted in less than a second and was then freely moveable to either display simply by resting the mouse pointer at the bottom of the desired display.