Radian install FF in Infiniti sedan -- ugg install

[hillview]

DS #1 is 39 lbs so I move his FF Radian to a belt install (was FF in the toyota minivan but moved the BV into the toyota) anyway.... the install on the Infiniti is ok but not great. NOT rock solid. It seems like the top of it (where DS's neck is) is loose. I have it tethered. It just doesn't seem like a great install. It doesn't go more then an inch each way but I am used to NO movement. Any tips??
Thanks!
/hillary