The only difficulty I've ever had was when the caravan was pushing or tugging against the car while trying to unhitch, i.e. the coupling was right against the end stop. If such conditions are unavoidable, such as on very uneven ground or when the need arises to unhitch with the handbrake applied, I run the jockey wheel down and use it to lift the caravan off the towball. In extreme case it will even lift the back of the car up before it parts company with the caravan but finally it's always done the trick. I've never had any problems if there was at least some degree of free movement in the coupling, though.