Double check that you still have the proper torque on the bolt. It is sometimes possible to make it to the torque spec without the bolt actually being at the correct torque or that it prematurely reached that torque because of damaged or dirty threads.

Perhaps the carbon paste you used had separated in the tube or container it was in and needed to be mixed back together. And all you got was just the grease or whatever vehicle the gritty stuff was supposed to be mixed with.