Depending on the surfaces carbon paste can work exactly opposite how it's supposed to. I'd clean out the seat tube and seatpost with a mild degreaser and then reinstall with both parts dry and with no carbon paste. You could also try another type of paste or even a dry clay assembly powder.

New clamp may not be as round as the old clamp which can contribute to showing a higher torque value at the bolt but with overall less holding power for the clamp connection.