Joie stages harness height

[Aurora94]

My 1.5yr old has the joie stages as a spare seat and I can’t seem to get the fit right, I’ve looked through the manual and I believe to raise the shoulder straps you need to raise the headrest as it does it together. I have done this but the straps are still far too low down and go quite far below his shoulders whilst the headrest appears too high.
Am I doing something wrong 🙈 I have the Britax romer maxsafe pro (I think it’s called 😬) in my car and it just seems to have a much better fit.

There is quite a gap between the position of the straps and the headrest in this seat.

For rear facing, up to 2cm below the shoulders is OK. It's better for them to be too low than too high because it helps prevent the child ramping up the slope of the seat in a crash (if that makes sense) - as long as they are not so low they are falling down the shoulders.

Do you have any inserts in still or not? I think the body insert would be OK to use still if it gives a better position.