Discovery 3 Tow Bar

I have a discovery 3 land rover and have a quick release tow bar. I have towed a swift conqueror twin axel and a elldis crusader twin axel and with both the vehicle bouncess at the back.......... Anyone else get this or can help I would be glad of your thoughts.

PS. I have got a nosewieght device and checked that on every journey ................ it seems to okay on the motorway doing 60??????????