Seatpost Length

I have set my seatpost to the correct riding height for me so my legs are extended to the right position. When the seat is the correct position the seat post can be rotated and the seat moves down when I sit on the seat because the seat post is short and does not extend to the seatpost clamp position to secure the seatpost. Riding with a seat that is not at the correct height can injure the knee joint and is dangerous when riding.

Has any of you eperienced the same issue? If so, what was your solution.