I've never had P-clamps not hold well. You're using the stainless steel ones with the loop dipped in black rubber? They stick like crazy. If you are using plastic ones, I'd dump them anyway because they tend to crack sooner or later and then all your hardware goes flopping into your spokes. I have fenders on a variety of carbon and steel forks and have no problems. You do have the right size? -- they do need to be tight.