Mine was the same, I swapped him to a 360 convertible car seat and it fits him perfectly, his head now stays upright and straight the whole time he is in it too!
Have you made sure they’re bum to buckle rather than bum to back of seat? That may help
@Freya what’s that one please?
Ours slumps too, I was told by the midwife its normal until they can hold their heads up properly, as long as it isn't restricting their airway, I guess its fine for now. This is probably the reason they aren't supposed to be in car seats very long xx