Hi,
If your power head has those three little cups, the vibration is what is making it not stay put, I don't care how smooth, or new the suction cups are. Now the one with the one big suction cup, with a clip is much better, and that one has to have a good smooth surface to make sure it makes good contact and then it will stay....When they came out with magnets to hold the power heads in place, I actually did a little gig.
You might try tying the power head in place, that's what I did until they came out with the better ones....from now on, make sure any PHs you purchase has magnets, or the one big suction cup with a clip.