Nup it shouldn't feel very wobbly or unstable. Try leaning or sitting on it like the other girls have said whilst you or your DH pull on the strap. Is the seatbelt tight arround it (if it's done that way), I always pull it really and the pull it quickly at the top to lock it in iykwim? If it's still wobbly maybe get it professionally fitted and ask them to show you how they did it?