Mine slid a bit as well. One thing I was careful about was over tightening the screws though. I think I did 10-12 twists on each screw and it was plenty tight.
I wasn't even sure it would fit in my case, I measured and knew it should fit but it was close. In the end I have about 5mm between the side wall of my case and the top of the heat pipes.
Intel I7-4770K, 8gb G.Skill DDR3 1600C9, MSI Z87M, Radeon 7850 2GB, Sandisk 240gb SSD + 1TB WD Black, Cooler Master Hyper 212 EVO