Maybe bend and "finesse" the tab a bit. It could EASILY have been bent when you pop out the black plastic plunger. If part of the plunger switch doesn't touch metal when you unscrew the head then there is no circuit, no current, and no light.
Troubleshooting reminder: Did you ever SEE this new light drop in work? What if it's defective? Did you know for sure? Try hooking 2 AA batteries directly to the module and make sure you don't get the voltage polarity backwards, it can fry a lot of LED modules. Make sure the dropin works before you pull out any more hair.Even if your switch works now, if the LED is DOA (dead on arrival,) no amount of switch fiddling will fix it.