Has anyone had difficulties using this as the high back booster. Once we realized the crotch strap was too tight for my son, we took out the straps and tried to use as the high back booster. It seems to be very difficult to secure and the seat doesn't fit well with the back seat design of my Chevy Impala. Very unsatisfied, I thought I was purchasing something that we could utilize a little longer. He only weighs 40 pounds and we shouldn't have to buy another seat yet.