It is a new bike? Or one with an extra-long seatpost? There are a few possibilities and solutions:

1. The seatpost is loose. Plausible if it's a new bike. All 27.2-mm seatposts are not created equal and all clamps are not created equal. If it's a new bike, see your LBS who might try a slightly larger seatpost.

2. The binder bolt is bent, broken, etc. and not doing its job. THat sometimes happen, especially if you like a tight fit. I just replaced the binder collar and bolt on my 2.5-year-old bike.

3. Bent seatpost or one with a "variable" diametre. If you sqeeze very hard the seatpost at the 10-cm mark, keep it there a good time, then raise it to 10.2 cm, is it possible to have a post sliding down to the place it was previously squeezed? Something might be measurable with a caliper.

4. Too much grease on the post. Yes, I know everyone suggests to grease the post to prevent it from sticking in place. I actually prefer non-greasy ones.

Regards,