When I used to use them, it was always a challenge getting the drive wheels on the ramps. Sometimes it took me 4 or 5 tries before I could get the car up there properly. Once I got so frustrated with them sliding I basically took a running start (of like, 5 mph) and ended up driving over one of the ramps and shooting it out the back of the car and into the back of the garage wall.
After that incident I started putting them on a big piece of cardboard, and it was much easier. I never had a problem putting the rear wheels up on my car, and my dad also used them a few times with his 4x4 and was always able to get it up easily, but it was a huge PITA to get the front wheels up.