Graco MyRide 65 Installation?

I just bought the new Graco MyRide, and can't seem to get this seat to fit snug enough rear facing. I've installed tons of car seats, but this one is tricky i guess. Has anyone else had a problem getting this car seat to install properly? I am using the seat belt (not latch). My problem is that the seat is not tight enough, and even when i push down on the seat i can't get it tight enough. I tried to use the locking clip, but can't get that to work. I'm frustrated lol...can anyone help? Maybe a trick to installing this seat?