This is indication of straps not being tight enough or wrong position. The straps should be online with baby’s shoulders or slightly beneath, never above. The head rest should start roughly above baby’s shoulders but you’ll be able to adjust that accordingly. Baby should have bum down so leg straps are actually on legs instead of hips, straps need to be pulled tight and make sure you can’t pinch the fabric. The shoulder pads should be tucked right under babies chin and that will usually hold their head up x
The seats not reclined enough that's why her head is flopping
Have you got a picture of the fit? Head flop is often a fit issue. Do you have the seat installed on the isofix base? Sometimes this can make the seat sit at more of a slant too x