I bought a Frost King (name brand) Electic Roof De-Icing Cable. But the clips provided that are supposed to attach to the shingles don't stay. They're flat pieces of metal... you bend over each end, and then shove one end under the shingle (the other end holds the cable). Pushing down on the shingle is supposed to hold it in place, but it doesn't. Is there another way to mount the cable on the roof, or to attach the clip so the shingle will hold it? Thanks.