There have been some issues with ENVE Seat-Post slippage it is very important to make sure that the hardware is installed and greased correctly.
I have attached a Page Link
that shows this process.
Please make sure that you refer to this when installing or adjusting the ENVE Seat-Post. The most common error people seem to make when they are having issues is that they fail to grease the wedge surfaces that slide on one another. This is where they must LIGHTLY grease. If they don’t grease the wedges they will bind and the customer will tighten down the bolt to what they believe is the proper torque spec when in reality they are just bound on the wedges and the clamping force isn’t being achieved.
The other issue quite common is over-greasing the wedges or getting grease into the inner diameter of the barrel. If there is grease on that inner surface the saddle will be more likely to slip for obvious reasons.