The seat post is not only supposed to be in to at least then the maximum height. The post need to be below the joints of the seat stays/seat post/top tube.

(disclaimer I quickly found these photos to help make my point. They link to were they came from.)

The Post needs to he bellow any of the points in the frame were it is joined together. I hope this might help. There is no seat stays on the bike but it still applies. The post should be bellow the the joint. It helps combats any leverage put on the frame with the seat post. This post should be farther in probably then what they are showing.



Or this might happen. I believe you can see the bottom of the post in the crack,(the silver bit in the left. It was not below the joint.