Car seat

I recently got the Saftey 1st guide 65 car seat. I installed it last night when I got home from work and it moves around in my car. I tightened it as much as possible, but it still falls backwards if I push on it. It's rear facing for now, but front facing, everything is fine, it's very secure. I tried the latches and the belt rear facing, do any of you know what I can do? It was a gift from my dad for my daughters birthday, he bought it on amazon so I'm not sure about the return policy. Thanks.