Can you adjust the angle of the car seat so that it doesn't happen, even just rolling a towel up in the apex of the back/seat part of the actual car seat (between it and the child seat) KWIM? Although both my child sears have a bar at the back of them that allows this sdjustment

Eta, I put some foam between the outer cover and the plastic of the seat of DS 's SnS seat because it had him sitting too upright, and he was really uncomfy in it. The foam is about 2cm thick and sits about 1/3 up the seat from the bottom. It just angled his bum a bit further forward and made him happier.