I have a set of these, they do move easily but I've had decent luck with them. Even able to get my 5spd Mustang up them without too much difficulty.
They have rubber pads underneath, mine fell off VERY easily. Might want to see if they're still there...they do make a difference. They do still move with the pads, but without the pads they'll be all over the place.
I'm also using these on rather rough concrete, haven't tried them on smoother surfaces, the movement could be alot worse, I could have an advantage with rougher surfaces!