Cutting MDF with Sculpfun S9

Hi
I am trying to understand the cutting speeds and Laser diode power.
I have a Sculpfun 9 and have cut many pieces of 3mm MDF at a speed of 300 mm/m at an intensity of 80%.
I then tried to make a jigsaw puzzle. I stuck the picture onto the mdf using a spray adhesive. I used exactly the same settings and it did not cut through.
the only addition is a piece of paper glued onto the MDF