The Solarforce bezel is deeper than the Surefire bezel, and might require a shim to make electrical/galvanic contact with the body/tube.
Check if the drop-in is stuck tight when the bezel is screwed down without batteries in the tube.
If there is a a slight rattle, you do need a shim!
This is also sometimes an issue with Surefire tubes, and drop-ins without the outer spring.