Did you try pushing the clips in really far? Sounds crazy but I have the square ones, and when we installed our moulded pad, even the square retainers would pop right back out. Then, our Daughter's b/f came along and pushed one in really far/hard and you could here it "pop", they have not popped out since.

Until he came along and tried pushing them further in, I was ready to throw the whole thing in the trash as I had just replaced the totally worthless "soft" hood insulation with the moulded one...