help! having difficulty unhitching the caravan

Hi, my wife and I are new to caravaning.  We have an Eldis 626 twin axle van (2009) and a LR Discovery 4 tow vehicle.  We never seem to have problems hitching up, but have had 5 sitations now where we could not unhitch.  The hitch is deployed all the way up, we have put down the jockey wheel and lifted the van, but the hitch and tow ball seem almost glued together.  We managed to get it off the last time by raising the cars suspension as high as it will go, placing a large tall stone under the tow hitch and then dropping the suspension again.  eventually it came off.  the wife described it as stuck on one second and then it wrenched free.  

the caravan is 3 years old, but we have only had it for 4 months.  We bought it from Glossop caravans who said it had been serviced.  

I have noticed that each time it happens the car has either been at a slight angle to the caravan or the ground has meant that the car and carvan are not straight.

Any ideas - it is really putting us off, stressing that we wont be able to unhitch at the destination.

thanks in advance

John