I recently got 'fitted' for my bike, and in the process, aquired a new seat post. It is chromed, and there seems to be a small amount of 'milling' around the circumference of the post. However, over the last two or three rides, the post slips down inside the tube. It's not a great dea;, 10-15mm, but it's noticeable. Is this due to a) the chromed finish b) a film of grease between the post and the clamp c) me being 110kg 0r d) all of the above. Any suggestions on how to make it non-slip?