I've had one slip and ended up sewing it to the quilt. I'm not sure if it was because the back was linty, or it had to do with the materials the machine was made of. At this point I can't remember if I was using it on my VX560 (all metal vintage machine) or the PC420 (all plastic casing) when it slipped.
I do remember that at the time I was not amused and ended up taping it down for a good while whenever I was FMQing--but it was a pain to get to the bobbin when it was taped down.
I've never had the slider slip when using it on my PQ1500s and I don't tape it down on that machine BUT I always make sure that it is nice and clean on the back before I apply it to my machine. BTW I'm still using the one that was sewed to the quilt, over the years it has developed quite a rip, but that doesn't seem to affect its performance as long as I put the rip to the left of or behind the needle.
That said, you might want to put it on the machine you use for FMQ and see if you can move it with your hands. It may be that the sliders stick better to some machines than others. Also, if you don't have good support to the left and behind the machine and are fighting against the weight of the quilt hanging off an edge, you could possibly cause a slider to slip simply by pressing down too hard as you try to move the quilt around.
Rob