A few different things can cause a vacuum breaker to fail. Consider the following to keep yours maintained and out of harm's way.
Installing the vacuum breaker wrong is the biggest reason why it fails. Typically, you will find that the fixture is loose and unable to work properly. If this happens to you, try to use a wrench or pliers to secure the vacuum breaker on the proper way so that it fits. Loost fixtures can also be caused by general wear and tear or even having the wrong size to begin with.
A clog is another common problem when it comes to the vacuum breaker. When something gets in the way, it won't work properly. To fix this, remove the entire fixture and use a toothbrush or scrub brush to clean out. Be careful not to do any damage. You may want to soak it in cleaner if you find that it is harder to remove any grime than it should be.
Sometimes you will find that the real problem is with the hose. If you have tried both of these things and are still having problems, try switching your water source.