As Richie mentioned, it's probably a battery length issue. Check the length of your battery, if it's much more than 65mm long, try another shorter 18650 cell.
If you're using a protected cell, it's going to be longer than an un-protected cell, and may not work in your laser.
A simple way to check for your maximum battery length for your unit, is to unscrew your tail cap until your switch works properly, and then measure the distance from a fully seated tail cap to where it is when it stars working; that distance is how much too long your 18650 is.
You'll need a cell that is at least that much shorter than what you have now.
BTW - Welcome to LPF, Robert, if you plan on staying, you may want to introduce yourself in the Welcome section. There is also a lot of useful information in the custom search bar at the bottom of the page.