Just like real joints in human body, they don't like to be overstressed. Make sure you don't apply too much force. And you should only change position and angle using Physics actions. If you try to change position or angle directly (using "Set angle" for example), or use other behaviors like Bullet, the joints will break.
If this doesn't help, please post your project file.
thanks IT happen always by torque to much, if stuck happen,i use remove joint,and create joint again,and i need know when stuck happen,very troublesome