Britax Froniter 85 Booster Mode Concerns?

I am finally giving in to my DD's demands and moving from harness mode to booster mode in the Britax Frontier 85. My DD is 7.5 years old and weighs 48 pounds and is 3'11".

When I put the Britax in booster mode it just doesn't seem to work with my car. I have a 2008 Chevy Suburban. I've tried in the 3rd row (where she sits now) and it doesn't sit flush with the seat since the block has to be at the back of the seat in booster bode. It is raised up in the front not sitting on the seat. And then when I buckle it, it keeps the slack out of the seat and you have to physically feed the seat belt through the red shoulder belt positioner back to where the seat belt comes from. The seat belt isn't locked like as in if you pulled it all the way out and it starts to click when it comes back. But instead, if she was to sit in the seat and move her body forward to pull slack from the seat belt and then sit back again against the booster, the seat belt doesn't come back in, it stays pulled out. So I moved the booster seat to an empty captain's chair. When it sits in the captain's chair it sits better against the seat (where the front of the seat isn't off the seat). It actually sits flush. BUT it still has problems with retracting the seat belt back in. It stays out. When she unbuckles, the seat belt just hangs there and doesn't go back in. When she moves forward it does the same without physically feeding the seatbelt back through the shoulder strap.

This doesn't seem right. I do have a Graco Highback Turbo booster that I purchased less than a month ago for her to some times ride in another car. I put that in the Chevy Suburban and it works fine in both the 3rd row (where I would rather her sit) and the captain's chair. I don't understand what's wrong with the Frontier. I have seen other people's comments when searching the forum but haven't found a solution... other than she might be too short. But I can't change that. And I thought since I paid a small FORTUNE for the Frontier it should work better than the cheap Graco Highback Turbo booster.

So my questions are:
1. Am I doing something wrong with the Frontier?
2. If I end up having to use the Graco, is it as safe as the Frontier? It seems that since it's so cheap, how could it be as safe? If it's not as safe, what would be a better option to buy?

P.S. We do travel on 8 hour long road trips monthly so I want everyone to be the safest!

I'd put the harness back in, use the Graco for daily use, and the FR for the trips. It's more reclined in harness mode anyway for sleeping. It's also nice to have a harnessed seat ready to go if she doesn't follow the booster rules. You can try moving the headrest up to see if it will work for her later on. DD always agreed to going back to the FR85 harness for road trips.

Thanks! I'll do just that.

I know she will be happy to be able to use a normal seat belt on daily trips.

To answer your questions nope nothing you can do to fix the frontier, it has to do with the angle of your seatbelt into the seat and her height which as you know you can't change. As long as the Graco fits her properly ( belt low on her hips and across her thighs and seatbelt across the middle of her shoulder) its just as safe. I like mommyshannons idea for trips