We had a Chicco carrier which was so simple for install. The convertible (rear facing) we bought for my husbands car seems more difficult and it took me longer to get it tight. The car seat is very secure near the belt, but wiggles a lot in the back. I can push the back up towards the back of the cars seat.
What kind is it? I've installed quite a few different seats and have always been able to get them tight if I sit in them. Are the straps twisted at all or caught on something? FWIW the chair will move up and down (well the part closest to the seat stays down), but shouldn't move side to side.
Having just installed a convertible carseat, I hear you on it being a PITA! We took it to the police station to get it inspected, and he had me kneel in the seat while pushing against the roof of the car with my back while he tightened the straps. He also said there should be less than an inch of movement at the belt patch/LATCH clips. He said the back of the seat will move up and down some, as it should b/c it's going to absorb the energy during a crash so it needs to be able to move a little.
I would get it as tight as you can and then take it to the police/fire station to get it checked.