I have actually lost a Powerbox fin once, but that was most certainly because of a bolt that was too short. I noticed that the threads engaged just a few turns of the screwdriver, but I was as usual in a hurry to get out.
And then I lost the fin...
I could imagine that it you shim with fincover material that is soft then you may get some play into the system, and that might work out (rocking it from side to side) the bolt eventually. I believe shim materials, if used, should be solid.
You also said that you "packed finbox with some fincover material at bottom". This could be the culprit. A powerbox fin is NOT intended to touch teh bottom of the box, it is supposed to be wedged in between the walls.
But I have never even heared about anyone else that would have needed to shim a powerbox fin.