Krill

Think I've solved this, if you have any other controllers attached to your machine (I have a Logitech G29 steering wheel) In the control options menu for the Mossie click the dropdown box for any other controller you have (apart from your stick & throttle) & select 'clear all categories' even if you have no key binds at all in there, then hit the dropdown again & select 'disabled'. This seemed to activate my trim controls in the Mossie. Even if you have other usb inputs attached & they are already disabled in DCS still follow the steps above, my steering wheel was already 'disabled' in DCS but unchecking 'disabled' then clearing all categories then rechecking 'disabled' again worked. Not sure about the reason for it but it worked for me. Hope this helps.

Think I've solved this, if you have any other controllers attached to your machine (I have a Logitech G29 steering wheel) In the control options menu for the Mossie click the dropdown box for any other controller you have (apart from your stick & throttle) & select 'clear all categories' even if you have no key binds at all in there, then hit the dropdown again & select 'disabled'. This seemed to activate my trim controls in the Mossie. Even if you have other usb inputs attached & they are already disabled in DCS still follow the steps above, my steering wheel was already 'disabled' in DCS but unchecking 'disabled' then clearing all categories then rechecking 'disabled' again worked. Not sure about the reason for it but it worked for me. Hope this helps.

Having EXACTLY the same issue you have described, trim having zero effect even though visually it moves in the cockpit & on the aircraft but it doesn't affect the trajectory of flight in the slightest, also playing in VR with a Reverb G2 & an X56. I have tried everything, bound it to an axis then bound it to a hat switch & even tried keyboard input. Nothing. Other than this everything works fine. It surely can't be a VR thing, can it? Strange.

I'm having the same issue as the OP, playing in VR (Reverb G2) with X56, elevator trim moving but having zero effect even if full trim in either direction.