Kiddy car seat - toddler slipping down

[BananaPie]

We've been using the kiddy guardian pro 2 seat for a few months now, but just recently our 18 month old has started slipping down in the seat so the impact cushion ends up level with her neck rather than across her chest. Has anyone else had this happen and worked out how to stop it? It's obviously dangerous once she's slipped, so we have to keep pulling over to adjust her. I make sure she is sitting right back in the seat before I put the cushion on, then pull the seatbelt tight. Can't work out what I'm doing wrong!

is she wearing a coat/think jumper? If so this could be stopping you getting the impact cushion in far enough. make sure she is in thin clothes when you put the cushion in and really wedge her in and pull the seatbelt tight

Have you removed the covers? We did and didn't put them back on "taut" enough so cushion arm didn't go far enough back IYSWIM.

There is a wedge shaped adjusting thing that you can get for them to sit on.

Thanks! Is the wedge thing something you can buy separately or is it the thing that came with the seat?

We've had a bit more luck with it this last couple of days, and not had any slipping incidents. Not sure what I'm doing differently though! We've never removed the covers, and she never wears her coat in the car as she gets quite hot in the seat. Frustrating that there seems to be a bit of a knack to using it properly!

I can't quite remember, to be honest, but I think it might have come with the seat. Our son sat on it until about 2.8, then we took it out.