Is this bad? Graco nautilus

We had a car seat tech come show us how to buckle in the graco nautilus's in my sisters 2012 Chevy cruize. One we latch which is easy the other is seat belt install & for the life of us we can't get it tight the way it is supposed to be installed. We have tried kneeling on it and sucking all he slack from the lap belt and then locking the top part of the seat belt but without fail every stinkin' time the lap belt had just enough slack to like turn or tilt sideways at the bottom. Is it okay to use the lock off (removable device that is on the back of the car seat) to get it to stop moving in a newer car? I thought the lock offs were more for older vehicles?

I mean we should be able to do this w/o the lock off and just the seat belt but can't

I have a 2013 chevy cruze, the nautilus is incompatible with a belt install.
I tried and tried and tried every single trick in the book, had some techs check it out too. It would appear to have a good install but one slight pull at the belt path at the side closest to the door and the nauti would slide right out. I ended up getting an evenflo sureride for my dd, and it is incredibly easy to install in the cruze with the belt (in all 3 rear positions, including that narror raised center seat). Plus, it has higher harness height than the nauti. I know it doesn't transition to a booster, but getting a hwh seat properly installed was more important to me, since hbbs can be bought later on ( There are quite a few hbbs' that I've tried that are incompatible too because of the forward tilted headrests). I also tried the harmony defender and that was incompatible too, same problem as the nauti.
the lock off you're talking about is a locking clip and is used to lock seat belts that don't lock, or they can be used to lock a seat belt with out using the belt's locking system to prevent seats from tipping by being pulled up on by the locked shoulder belt. It will not help with the nauti install, the problem with the nautilus and the cruze is the geometry of the seat does not work with the geometry of the vehicle's seat and seat belt. I would also double check the nautilus that you think it properly installed. We were not able to get the nauti installed properly on either side