Buckle crunch is the term used when the vehicle's buckle is so long that it starts to bend round the framework of the child seat, causing extra pressure onto the vehicle buckle which can potentially cause it to break in the event of a collision. Check to see if your seat has an alternative router.
Some seats like this have a second router so if buckle crunch is found using the primary router, the buckle can be passed through a secondary route which allows the buckle to be pulled in a nice straight line taking the pressure off it allowing it to fit safely to the vehicle.
By moving the seat from the side seat which is normally dipped to the central seat where it is raised, this will often move the seat further up and away from the potential buckle crunch issue. If there is still a problem, you can try the front seat providing there is no airbag or the airbag has the facility to be turned off.