Baby poorly fitting in car seat

Hi I was just wondering if anyone could give me some advice or if a similar thing has happened to anyone else. My baby fitted fine in the mothercare journey car seat with all the padding from birth but now she's a Petit 6 month old is too squished with padding but seems not comfortable in the seat atall she hates it most of the time and now I'm worried it's hurting her head because the lack of padding on the seat all the holes for the different stages of belt show and are bumpy? I don't know whether I'm just over thinking or I should just buy a from birth to 4yrs seat because it will be comfy through all the different stages. Thank you in advance.