I have never used carbon frames or components, but I've had similar issues with aluminum posts in aluminum mountain bike frames. In all cases the clamp was a hand tightened quick release type. I also helped friends with this issue, some of them had carbon frames or seat posts. Possible causes and correction applied where as follows:
Clamp is not providing enough clamping force. Adjust the clamp. Almost always this kind of clamp has some quick release lever in one side, and a corresponding bolt or nut in the opposite side, tighten it a little bit at a time until problem disappears.
Seatpost or inside the seat tube is dirty or oily. Debris, sand, mud, oil or even water between seat tube and seat post will create this kind of problem, also accelerating wear of both even when no apparent slippage occurs. Clean both and make sure they are reasonably dry before putting them back together. How does all the stuff get in there? mainly the rear wheel trows it, it sticks to the seatpost and then, during cleanup it runs down into the seat tube. Some frames are more prone than others to collect dust. Shall this be the case, there are commercially available neoprene or rubber protections that seal the clamp area, but on the cheap you can use scraps from old inner tubes.
The quick release lever is producing too much friction: due to dirt and/or damage or wear, the quick release may be developing too much friction, giving the false feel of tightening, but the force you excerpt is being wasted. Inspect the clamp and be sure it is not dirty or deformed, specially around the quick release pivot. Friction surfaces of the quick release and their counterparts shall be clean, smooth and dent free. Some clamps have a plastic, teflon or steel "cushion" that help these friction surfaces slide. This must be well shaped and clean. The friction surface of the quick release lever can be lubed with oil or light grease. That also helps reducing the force required to securely tighten the seat post. Dents and small deformations can be sanded down and polished with very fine grit sand paper.
There is dirt caught between the clamp and the neck of the seat tube: Dirt or other materials caught in there can also prevent clamp force to properly tighten the seat tube. Clean it, the the inside of the clamp and remove any stickers or loose paint. (Special attention with repainted frames here) The inner surface of the clamp can also be lubed and it helps, but very little.
Clamp is misaligned: The seat tube has a cut part in the neck, this should be aligned with the clamp's gap (Unless some weird manufacturer says otherwise). Check alignment, special attention if the clamp is an aftermarket one.
Wear: If your problem has been happening for a long time, chances are the seatpost or even the inner seat tube is worn out, thus having too much "play" between them and being too much flex for the seat tube to take up, so tightening it is not effective. To be sure seat post and seat tube must be measured with a precision caliper. Shall wear be the problem, hopefully changing your seat post is enough to solve the problem.
Never use excessive force to tighten the seatpost (or any other component), specially with carbon components, you'll cause too much stress and reduce the time to failure of the component.