It can also depend on where on the back seat you are trying to install it, because it has a wide base it does not sit well in the centre of the rear seat of many vehicles as the centre is often narrower and slightly raised, I found it needed to be in one of the side positions that are wider and have a slight dip to fit well.
I had this thought. Our middle seat is quite narrow, and the seat belt is also a bit odd because we drive a wagon. I know it's safer in the middle, but not if its moving I guess!
Does it move more than 2.5cm side to side if you push it at the belt path? Also, sometimes in FF mode a seat will install better in upright mode rather than reclined so try that too.
I think it does move this much. It's currently in reclined position, so I'll try it upright. Will also try the towel and the side seats before calling someone out. Thanks all