Make sure the plugs are screwed-in tight. If they are loose, they could be leaking some compression into the space where the coil fits and be pushing it out. |
Also, I noticed when I changed my plugs, the coils gripped the new ones a lot better than the ones I pulled out. I think the threads on the old plugs get worn down a bit after a while and the coil doesn't grip on as strong. The new plugs definitely were gripped a lot more firmly by the coil.