scrod I'm having issues with my compression plug on my carbon fork.

ill tighten it down, make sure its not too tight and it'll feel good and I'll head out. the first big pot hole I hit I will hear a 'ping' and then the cap comes loose enough that I can turn it by hand. I tried adding another 2.5mm spacer on top to see if that would do it, but it's still happening. any ideas?