Suggestion on convertible car seat

My LO is 12 months. She is still under the height and weight for her infant car seat (uppa baby Mesa) but she has started to unclip her chest clip and she seems uncomfortable. Not sure what else to do besides move her up to convertible seat.? We picked the uppa baby Mesa because of how safe it is and green. We have a Jeep Grand Cherokee for size reference. Any suggestions? Thanks!