P.A.
I have begun to believe that comfort in car seats will vary by body type. I've had to change out car seats over the years, for my 3 children.
So my suggestion, after you do your saftey checks, take your daughter shopping with you. Let her try some seats out and then let her pick. You'll be able to see what fits her body best and it will give her ownership of the seat, and therefore help any other issues of not being a good car seat rider. You may even find she likes the seat you have but that a new seat cover will satisfy her.
Have a safe trip.