Hey Bertie,
The longer boards dont release well except from the tail. I had smilar problems with the Laird 12 fter.
On take off you need to step forward to get momentum down the wave and then get back fast to make the bottom turn.
Normally the fin the supplied has to much base area and this makes the board track on take off. Using a more upright cut away fin will assit in this area.
Also with the Mistral you effectively have a large longboard with lots of swing weight, so you need to get back as far on the tail as you can. Your back foot needs to be over the middle of the center fin for the board to release properly other wise you will pull a rail all the time.
Put a line of wax across the the board from side to side to locate your foot or something you can feel that will locate your foot in the right spot.
The lighter weight you are the further back you will need to get.
Hope this helps.
Phill