Laser head will not go all the way down on the mount

I havejust got the Falcon 2 22w and when mounting the laser it will get to where the bottom of the laser module is level with the botom of the mount and it gets stuck and will not go any further down (without excess force) the thumbscrews are not engaged and have even removed them fully.
I have a 1.6w laser supplied to and that will slide up and down easiliy in the mount and able to move down to meet the surface.
I could attack the diod and mount with some sandpaper but wanted some advice first.
Thanks